What I expected to happen was...:
I should have been able to swim to the surface of the water.

What actually happened was...:
I was unable to return to the surface. Although in creative mode flying gets you out of the water.

Steps to Reproduce:
1. Dig a hole that is a square 2 blocks wide and two blocks deep
2. Fill it with water
3. Enter the water and sink to the bottom
4. When trying to return to the surface you will be stuck underwater