It might increase the difficulty of the encounter one step.
There is some guidance in the encounter design section of the Dungeon Master’s Guide that is relevant here:
Increase the difficulty of the encounter by one step (from easy to medium, for example) if the characters have a drawback that their enemies don’t. Reduce the difficulty by one step if the characters have a benefit that their enemies don’t. Any additional benefit or drawback pushes the encounter one step in the appropriate direction. If the characters have both a benefit and a drawback, the two cancel each other out.
In your scenario, if none of the party have a swim speed, the encounter taking place under water would definitely count as a drawback here for the purposes of estimating encounter difficulty. The Chuul, having a swim speed will find underwater combat to be as easy as combat out of water, your party however, will be at a disadvantage:
When making a melee weapon attack, a creature that doesn't have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident.
A ranged weapon attack automatically misses a target beyond the weapon's normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).
On the other hand, if the party would be mostly unimpeded by fighting underwater (by everyone having a swimming speed), fighting underwater would not affect the encounter difficulty at all.
