OK all - talked to the seller last night, and he taught me a bit more about the mangroves that I thought I'd share.
The leaves DO need to be out of the water. They should NOT be fully submerged. They can be tied to acrylic rods that are stuck into your sand/substrate, if you need to elevate them. You can also cut a piece of styrofoam to fit them into, which will float them at the surface and allow the roots to reach down into the sand. Once that happens, and the roots can stabilize the mangrove, the rod or foam can go away.
It looks really cool when the roots grow all the way down from about halfway up in your fuge, because they form kind of a cage that seahorses and fish (smaller ones) can swim in and around. Just my opinion, anyway.
The higher the leaves are out of the water, the less salt spray will get on them, and you won't have to mist them as often.
They will do MUCH better if they are stationary in the water. If they are moving around, the plant won't grow roots or leaves as quickly. Apparently mangrove seeds/seedlings know when they're not stationary, will remain dormant until they stop moving. Once they are stationary, they then sprout, grow, and take root. He sent us young plants so that would more easily adapt to our tank conditions.
The seller guarantees these for 2 months. If you have any problems, contact him directly and tell him you were part of the Denver parrothead's group buy. That'll make him laugh, and he'll know exactly who you're talking about. His name is Jim, and he can be reached through his ebay listings. He's a very helpful guy, who can probably answer any questions you might have about mangroves.