when jumping in water the water blocks act as air and damage can be taken from far falls (water blocks act like air but still visually look the same and account for breathing. getting out of water is the same as climbing up or down a hill/mountain.
using a boat in water: placing the boat it floats, once you get in the boat it sinks, getting out of the boat it remains an the level you got out and if you break it, it will then float to the top.