[ScubaSteve]

You can completely submerge them like some people do, however, they don't appreciate it. While they are capable of removing most of the salt from the water as they absorb it, some salt still builds up inside of them; because of this mangroves appreciate being partially in the air so that they can excrete the excess salt on their leaves or pump the salt into some sacrificial leaves which then fall off. Air is needed for these processes. Remember, mangroves grow in tidal zones; so they're not submerged 100% of the time.

I'm not saying you can't completely submerge them but, like your corals, they have their own specific needs and access to air, for atleast part of the day, is one of those needs for a healthy plant.

Also, from one of Anthony Calfo's papers (I can't recall which one it is at the moment) he says that for most mangrove species he only observed significant growth when the leaves were spritzed with fresh water every few days. He said that it is likely because of the same reasons I wrote above and he hypothesized that the spritzing "washes" the salt from the leaves the same way a storm does. Something to consider especially if you are wanting to keep them in your display.

I've been wanting to do a mangrove display tank for a while (hence why I know this crap). One idea that I had was to build up the back of the tank to create a ledge a few inches under the surface and creating another sand bed up there to plant mangroves; that way you get a cool shallows/wall/reef set-up all in one. You'd probably need an acrylic box to hold the sand and disguise it behind some rock. Haven't had the $$$ to do it yet though.