This is something that I like to do...it gives resistance on both the extension and retraction of the kick, or punch. If you have a weight bearing injury you can be partially supported by the water at the same time that you are resisted by it. Your footwork will get more explosive...however there are some oddities, i.e, if you step and punch in neck high water the punch will be much faster than the step. So really you have to practice the techniques standing still, and the footwork separately. I also like to practice with my snorkel on, completely submerged, or you can do the hold your breath like the tv dude. Water training is also good if you have bruising on contact points ie, knuckles, shin, you get more resistance than shadow boxing, but no impact.
Oh, yeah one more thing... longtime ago, my instructor and i used to go to the ocean, stand in crotch high water and practice thai round and thrust kicks to waist or head high against the pads... it really gave you a sense of accelerating through the kick, plus the added incentive of trying to knock your partner underwater..