In Angel of Darkness
Boiling Water is a trap in the level Eckhardt's Lab in Tomb Raider: The Angel of Darkness. There are two locations of the boiling water: one is a circular pool and the other is a pit with a dangling cage on it.
While searching for two of the remaining Alchemic Phials, Lara must use a cage to lower herself to the pit in order to reach the openings with the phials. One must move quickly while in the pit, otherwise Lara will sink to the water alongside the cage.
After placing all three Alchemic Phials to their receptacles, some glowing substance will flow into the circular pool of Boiling Water. This cools the water down, making it possible to jump into the pool and reach a Periapt Shard.
In Temple of Osiris
Boiling Water in Temple of Osiris is heated up by the Gas Vents Lara and her companions have to light in order to progress through the game. To avoid damage, one has move quickly after lighting the vents with the Torch or try to stay out of the water completely by using the Flamethrower to light the vents. Once heat up the water will damage the player and possibly even kill him.