Spawn a soldier near water. Notice that if you crawl into the water, as you go deeper the solder will crouch and then stand to keep their head above water level. If you go prone on the shore and roll into the water, you can remain underwater for as long as you remain in water shallower than standing depth and keep tapping a roll key.
Stopping rolling or going deeper than standing depth will transition the character to standing or swimming (depending on depth).
Pinewood lake is a good test area. Works in the ocean too.