Never heard of it. Dock-o-miniums are common though. You own the bottomland under your slip, and sometimes your slip itself, and the "condominium association" owns and maintains the common property, i.e. the main docks, parking lot, buildings and other stuff.
Sometimes the ownership is outright, sometimes it is a 99-year lease...and like any other group ownership scheme, you really need to look at the complete financials for the whole group before you buy in.