On our OpenSim server, my kids are building underwater villas and a submerged cinema. Quite a weird thought one would say.. Yet, after seeing the results, I thought of something else: Second Life would be a great environment to recreate an underwater archeology site! Most of us never get to dive to the real thing, whether they be famous shipwrecks or lost cities in, for instance, the Mediterranean. From a promotional or educational perspective, this could be interesting!
I own some real estate in the Fortimus area; I centered a few rental houses around a water parcel; this way, every rental has open water access. Maybe I should try to create something like that, there. Too bad I'm a lousy builder :-)