A somewhat neat little trick that's probably a glitch.
To Crouch Anchor, you must be at either a ledge which is right above water or in a location where water will rise over it while you're standing there. The latter one is easy to do; all you need to do is stay crouched. For the ledge, you'll need to run over the ledge and crouch just before you drop off. If you do this correctly, you'll be crouching while falling into the water. When crouching in water, the game won't register you being in the water, and you'll basically be "anchored" at the bottom of the water as long as you stay crouching. Also, Crouch Anchoring protects you from dangerous fluids such as the hot water in Lava Lagoon and the poisonous sludge in Toxic Tower (as long as you stay crouching; it'll hurt you if you attempt to swim afterwards.)
There's another interesting trick you can do with Crouch Anchoring by pressing the B button while you're crouching in the water. If you do this, you'll swim up really quickly.