It's actually a garage for SDVs, which would be stored inside, safe and dry, until they're deployed. All current SDVs are "open air" models. The only real minisub was the ASDS, which was removed from service a few years ago.

I've qualified non-Navy operators for lock out/lock in, but that was mostly just a boondoggle. For actual operational purposes why would you use anyone other than the experienced pro's? You can imagine a few very rare exceptions.