Search the forums. I've suggested basements in the past. hehe
With houses the z index (height) doesn't matter so as long as the basement doesn't extend beyond the edge of the house plot you can still lockdown and secure things. Glad to see others are interested in this idea.

As far as boats go, I'll have to check out the big one you are talking about. A huge ship would be kind of cool.... especially if you could lock down stuff on it and it didn't decay. Then it would be like a mobile house. hehe