I like the concept, but the walkways would be problematic on the high seas. As we can see, none have survived to present day. The walkways would be a major weakness.
I'd like to put one out on a lake I owned, with the fort about 30-40 feet above the waterline, and something like this below the water:
http://www.ussubs.com/habitats/index.php3If I'm gonna dream, I might as well dream big. The lake would have to be relatively clear so that the underwater habitat would actually be a nice place to live.
Think of how quiet it would be, say, 30-40 feet down, with light BARELY filtering in through the water...
S