Remember doing some submarine work at the N end of Exuma Sound. Sub had a 'rattle' and we were tasked with finding and fixing it. Aft end of the sub has zincs on the exterior for corrosion protection, held on by studs and self-locking nuts. These zincs are about 6"x12"x 1" thick. One diagnostic was for our diver to whack these things with a rubber mallet, and if they rattled they were either tightened up or broken off. The diver broke one off and dropped it. Noting the clarity of the water, he timed the second broken off zinc until he couldn't see it - 25 seconds.
Underwater, our divers had no trouble spotting sharks at 100 yards - the sailors on deck with the rifles couldn't see them as far due to surface waves. But the sailors didn't seem concerned as they weren't Navy divers - just contractors. Getting our divers and gear to Eluthera was interesting - particularly the SCUBA tanks (which can't be pressurized for flight). Once you get all this stuff through Bahama Customs, then you have to get the tanks filled. And Bahama Customs is an interesting group, interested in one thing - import duty. If something you're trans-shipping (ie: to a submarine) ends up in a bonded warehouse, getting it out without paying duty is way beyond my pay grade. Sorry - I'm starting to digress...