When under 6 or more blocks of water, you can no longer swim upwards

When under 6 or more blocks of water, you can no longer swim upwards.

What I expected to happen was...:
To be able to swim to the surface.

What actually happened was...:
I made very small and fast jumping motions and never came up.

Steps to Reproduce:
1. Go into non-flowing water.
2. Go down 6 or more blocks deep.
3. Try to swim up. (Hold spacebar)