When swimming within bottom stairs or slabs, the water shader does not appear.

The title says it all. Whenever you're swimming in a lower, bottom, or whatever else you want to call it, half block, trapdoor, etc, with another half-block above your head, then the water shader does not apply.

This is likely because the game thinks your head actually outside the block - which you can see by shifting or going into f5.

A short video showing this is attached.