I have no idea of the US Navy's reason, this is just a thought. As stupid as it sounds, is it possible they don't want any controversy over where aircraft they recover go? Like I said, extremely stupid idea, but I've heard stranger things in the past!
Too bad about the Lake Erie Boly, my Dad told me about when he was young and at their cottage on Lake Erie he saw it while scuba diving. That story fueled my imagination and my dreams of one day finding a wreck and restoring it. After that I learned about the time, money, effort, political nonsense etc, which has only slightly dettered me
This may be an interesting site to check out, it's the Canadian Historical Aircraft Association Dive Recovery Team, not only looking for Harvards, but any wrecks.
http://www.chaa-recovery.ca/
What is this 4 engine bomber? Sounds like there'd be an interesting story behind how it ended up there.