When you are in a one block tall hole and you swim in it and block the source block(s), you can still swim. However, when you do move, you go extremely fast.

Steps to reproduce:
1. find ocean biome
2. find somewhere you can make a 1 block tall hole in
3. dig a 4x4 hole that is 1 block tall
4. go in the area you just dug
5. block the source block(s)

Expected Results: You don't swim.
Observed Results: You still swim