The players may have gotten out of the boss fight I had intended

Excellent - I love it when players outsmart me, giving players the chance to feel clever and empowered is what being a DM is all about.

Victorious warriors win first and then go to war, while defeated warriors go to war first and then seek to win.

Sun Tzu

Take a step back and ask: what is the fundamental purpose of this (or any) encounter? I submit that it is to challenge the players. Specifically, it is to challenge them to manage their characters’ resources intelligently and efficiently - D&D is, after all, a resource management game.

You thought the encounter was a combat encounter, the players, correctly, saw it as a trap.

In this sense it was a successful encounter - the players expended time and spell slots to overcome it. A more efficient solution than the one you imagined; you expected them to expend spell slots, hit points and class abilities.

That said, you have learned a lesson - slow moving melee monsters need to be used as ambush or trap using predators rather than pursuit predators.

Add allies that move, fly, or have ranged attacks

Your players employed good tactics to deal with a particular challenge. Using ranged attacks is such a great advantage in combat that in real life, ranged weapons keep on getting better over the years.

Since you want to challenge your players, you as the DM need to set up a situation that forces them to make another tactical choice: focus fire on the slow enemy, or deal with that quick / nimble enemy, or try to do both. Each choice has a consequence.

Example: the group I DM for were battling a horde of zombies, and two zombie ogres, but there was also that nasty leader of this group, a wight, who was shooting at them with a long bow. Wights have some rather nasty side effects on successful hits in melee, but this one was trying to put the spell casters down first to then get into melee and make another zombie out of a PC. The party had to figure out how to break up the synergy, since the intelligent wight kept targeting the spell casters with his bow shots.

As it turned out, the party's cleric turn undead attempt was so effective that the bulk of the zombies ran off, leaving the party with a less difficult problem to solve. If fewer undead had missed their wisdom save ...

Back to your problem: single monsters are susceptible to "focus fire" from the entire party. Give them an ally, or a side kick, so that the party has more than one problem to solve.

If you've got a group that are using creative combat solutions you are going to need to do the same thing to challenge them.

In general, a slow melee monster needs to contain or immobilise faster opponents. So you want to think about setups that trap or corner the players with the monster. How that is set up will be completely dependent on the situation.

In the example you give, it sounds like draining the water would take a very long time using their method.

1. What else is in the area that might wander up behind them,
   sandwiching them between the Chuul and the wandering enemies?




2. What was the Chuul doing all that time, aren't they fond of
   burrowing and ambushing their prey?




3. Would it have a network of tunnels throughout the the area that it
   could use to pop up behind the players, trapping them between it and
   the water?




4. Is there a pit that it had burrowed out that it could collapse under
   the players?

Now the encounter looks something like this:

The party is sitting around playing dice while the magic users painstakingly haul blocks of ice out of the water. The ground under them begins to vibrate and suddenly collapses, dropping them into a deep pit (roll to dive to safety) with an angry Chuul. Also, the pit is rapidly filling with water pouring through the tunnel it dug from the bottom of the lake. Roll initiative :P

It's good that your players are using tactics, just remember you can as well. Players will always try to fight safely so it helps to design encounters that make that less attractive.

You have lots of options for keeping fights close.

1. Ambush: predators and villains want ambushes so have them set up for them. Spider webs, burrows, camouflage, there are many ways even animals can set up good ambushes. For intelligent enemies the sky is the limit.




2. Traps: a trap can dump you in close proximity or make staying back dangerous. My personal favorite is the tilting floor seesaw trap, since the players can't back off without setting off the trap and dumping them in even tighter quarters.




3. Room design: doors that lock behind you, ramps, tight winding passages, all favor close combat. Castles were designed not to have long straight passages for this reason, since even something a simple as a right angle to enter a room forces close combat. Even better is a winding labyrinth plus incorporeal undead -- remember that down into the floor is an option for the incorporeal. Don't give them big open areas to fight in.




4. Intelligence: intelligent enemies will use tactics as well. The players back down a hallway, and the enemy does not follow, but instead moves out of sight of the door and starts buffing themselves, then draws their own ranged weapons or readies an action, or better yet goes for help. Intelligent enemies can use any tactic the PC's do, often even better ones since they are familiar with the local terrain.




5. Ranged combat: monsters can do ranged combat too, some better than the players, so don't be afraid to add a few to an encounter. Gaze attacks can be very good for this. You can add things like cover and higher elevation to give your monsters a bigger advantage.




6. Speed and teleportation: many monsters can close the distance in a round, then when the players try to back up they get hit by attacks of opportunity. Monsters with reach work really well for this.




7. Groups: whichever party has more people has a distinct advantage in 5e. You can easily use that to your advantage by having the enemy in larger groups. A group of mixed creatures can make retreating or spreading out bad because it lets a group of monsters focus on a single player. Works amazingly well if one of them can grapple.




8. Goals and time limits: civilians in peril, a dark ritual, or villains who retreat. The players can't play it safe if they are on a ticking clock. Enemies that flee to get help are also good for this.




9. Antimagic field: takes away your distance big hitters, add cover and you really have to focus on melee. There are enemies resistant to magic or piercing which is the bulk of the ranged damage.

Since no one else mentioned it:

Dash action.

With dash, you get twice your movement for the price of action. Now, Chuuls are no sprinters, but 60 ft of movement should be enough to close the distance to the party in one or at most two turns. Yes, the heroes can do the same, but that means they miss out on attacking (and you get a nice dramatic pursuit for which may even want to consider the available chase rules – DMG p. 252). Once in melee range, you get at least an attack of opportunity, if they try to flee.

Moreover, Chuuls' pincers can graple, ensuring the hero stays in place.

I am well aware that this may not be an optimal engagement – this answer is mostly for the sake of completeness and for the benefit of the newer DM, who might easily forget this simple, generally applicable solution.

Make Time a Real factor

You shouldn't have a problem with players being tactical. It's interesting. The problem is that they are taking an unreasonably long time to do something. (How long did it take them to empty the pool with Shape Water and Mage Hand? Hours? Days? Weeks?)

You counter this by putting some small time pressure on them. Make them check their food rations. Have any NPCs complain about how bored they are getting, or have them wander off down the corridors in search of something more interesting. Or start a poker game while they are waiting, with the inevitable consequence of a fight starting when someone cheats. Have occasional other monsters wander into the area - not ones strong enough to kill the party, but enough to keep whittling their hit points down a bit, or forcing them to use spells. This is what would happen in real life, or in any self-respecting fantasy book.

This doesn't mean you have to start tracking time all the time (ahem) but just when players are treating it like an infinite resource to be squandered.

The other answers cover most of the relevant items, but I would remind you of two more things:

• Dodge Action - Any creature who can't practically use their Action to attack is well served to utilize the Dodge action in order to buy time as they attempt to close the distance.


• Cover - Using the Dodge action in combination with cover is always a good idea. Cover is a broad category and there's a lot of things that can fit the bill including stone columns, corpses, or whatever else the Chuul can put between itself and the ranged attackers.

Your monster has a base AC of 16. If they can practically find a means to provide half-cover and use the Dodge action, they can bump that up to 18 and impose disadvantage on all attacks against them. As a result, a substantial number of the player attacks are likely to miss while the Chuul chases them, presumably to a location that is more confined and suited to its talents.

While many good answers have gone down, I'll try to give a few generic-as-possible approaches.

Surprise

If the characters don't know the melee creature is there before being in melee, they don't know to avoid getting close. They might try to get back out, but you did get the big guy in melee to begin with.

Cover

If the melee creature is behind a wall and unwilling to come out, and there's no place around the wall that is still away from the creature, the players either have to flush it out (perfectly possible) or get close. Imagine a monster just around the bend of a corridor: the only place that can get an arrow around the corner is close enough to get mauled.

Support Fire

A favorite among video game developers for a reason: the melee brute offers threat, while the archer behind it offers urgency. The characters can deal with the archer slowly, forced at range by the melee threat, at the cost of being fired at. They can take even longer to defeat the brute at range, while still suffering ranged attacks. Or they can attempt to tangle with the brute, if only enough to get through to the skirmisher.

A combination of the above

What you really should be doing. Particularly nasty in 5e is a combination of cover and support fire, owing to the fact you can attack in the middle of your move and cannot ready an action (such as an attack to hit the archer when it emerges from cover).

In your case, for example, a pair of CR1/4 skeletons with shortbows, hiding behind boulders on either side of the room across the Chuul's puddle would force reaction from the characters: on their turn, they'd emerge, fire (low-damage, low-hit-chance, meager) arrows and hide again, being safe until their next turn. That would get your players faced with a dilemma: suffer an HP tax for every round they spend ruining the puddle, or charge the Chuul (or past it, to get rid of the archers and go back to their careful approach - at which point the Chuul has had its chance, as the characters fought through it).

Add a condition that forces some kind of trade

I'm not a very experienced DM, but I've had some success with enforcing a trade-off for taking the cautious approach. Whether that be a dwindling resource (or a diminishing reward), an ongoing ritual of unknown duration, an adversary making their escape, or anything of the like I have found that it forces my party to make a decision on whether they value speed or caution more at that moment.

They can proceed more cautiously (read: slowly), conserving resources and potentially cheesing their way out of an otherwise difficult fight (much as your party did), but then their treasure will have been carted away, or their quarry will have escaped, or the ritual is completed, and so on.

Or, they can rush forward and not give any of that up, but in doing so will place themselves at greater risk and possibly get them ambushed or separated. And of course, a wizard rarely wants to be separated from his meatshield- I mean, from the tanks!

Now of course, it's D&D, so there is always the chance that the party surprises you somehow. In those cases, letting them have the win does a few things for you.

First, it makes the party feel good about themselves!

Second, it makes a fun story for later (and that's what keeps people coming back)!

And third, it shows your party that you're on their side, even if you occasionally throw something in their way!

After all, the point's to have fun and tell a good story!

Internal Consistency helps

Whatever condition you come up with to increase the challenge, try to make it seem like a natural consequence rather than a contrived scenario. I've found that my players don't like it if I seem to be arbitrarily penalizing them for something, but they'll accept quite a bit more if I can make it a natural part of the story.