As someone who read a couple of books about ships once and was interested enough that I wanted to research (in the current, less strict meaning of the word) this upon seeing the question, I think I can help.
I assume a submerged McMansion is basically a shipwreck (I am not a marine lawyer), in which case you don't strictly need to wait (I am not a marine lawyer), a person who voluntarily rescues property from marine danger is entitled to fair compensation (I am not a marine lawyer). It's still technically the property of the original owner, so they may prefer to give you money or some other asset in lieu of the goods themselves, but there's something in it for you (I am not a marine lawyer)! If you can't come to an agreement, you may have to take it to arbitration, be prepared to consult a marine lawyer.
You do have to be careful not to leave it too long. In the US once a wreck becomes embedded, which is kind of ambiguous but basically means that you can't access it without shifting the sediment (or coral), it belongs to the whichever state you're in, and you're not allowed to touch them at all.
Best check your local laws unless you happen to be salvaging in Bir Tawil (unlikely) or that one chunk of Antarctica (more likely). If I had to give one piece of advice, it would be to speak to a real marine lawyer before going any further. Then keep him on hand in case you need that arbitration.