If the water height is set to 128 or higher, the water becomes "Walk in water", which is a sembalance of no gravity. If you hold down jump in it, you go up for a very long time, usually until you hit the ceilling, or get out of the water. But eventually, given a chance, your jump does stop. Then you fall, slowly at first, then gradually faster. Double jumping in this water restarts your jump, only it takes a bit longer before your jump strength runs out.