|There is a pressure plate under the water that, when depressed, causes the water to magically freeze.
A DC 18 Wisdom (Perception) check reveals a few small air bubbles coming out from a tile under the water about halfway down the hallway.
A DC 12 Intelligence (Investigation) check reveals that there is a small air pocket under the tile, which can be moved down.
When the tile is stepped on, each target in the water must make a DC 16 Dexterity saving throw, or become trapped in the ice. Targets trapped in the ice take 3d6 cold damage each round, while those not trapped in the ice are standing on top of it, and take 1d6 cold damage per round.
Targets trapped in the ice must break free. The ice has AC 12 and 10 hp. It is immune to cold damage and vulnerable to fire damage. Once destroyed, they are free to move.
The ice lasts for 10 minutes.
If the tile is discovered, it can be jumped with a DC 10 Strength (Acrobatics) check at disadvantage due to the water.
It can also be disabled with a DC 14 Dexterity check with thieves’ tools at disadvantage, due to the water.