I was thinking prank as well...but these days...four people...dress all in black...wandering around on a military ship before dawn...
I know the layout there, and it astonishes me that anyone can get aboard the ship off-hours without being stopped or even seen. I imagine the Naval Museum is going to tighten access even more and put physical barriers up.
The dressed in black may be interesting or maybe not.
It could indicate a group or cult or it could just indicate burglars dressing so as not to be detected.