The blind man

Ok,notajoke; what I want to know is ;1) how do those things float, and 2) why are there triggers on the underside, I mean, what are the chances that a sub would ever enter a minefield..
Any water tight container will float. There probably is some kind of long drawn out explanation as to volume density and blah blah blah but what I don’t understand is if the oil tanks on an oil freighter aren’t broken how could it sink oil lighter than water.