When I was putting out a fire I started, I was running around and sprinted into a puddle. When this happened, the lower half of my avatar clipped into the block below the water
I tested this further, it works for all water levels no greater than 1 block (1-7), untested on 2 block high water
Recent snapshots introduced a swimming mechanic, which is activated by using the sprint key in water. Press F5, and hopefully it'll make more sense.