I thought the skull and crossbones was the battle flag of the fleet of the Knights Templar, back in the 13th/14th century. They were last seen leaving Paris with the contents of the Templar treasury the night before the arrest of Jacques de Molay, the Templar Grand Master, and all the rest of the Order in France.
I believe that it is tradition with the submariners that after sinking a boat they fly the "Jolly" when entering port. There was a very interesting program on the box a few weeks ago about the admiral who run the submarine fleet during the second WW who started the tradition, errr I think, that is if I was awake, or was I dreaming this !!
I don't think you could get a sub up the Ouse, it's only 5ft in a lot of places now (no dredging and no commercial traffic, people wonder why we are getting more floods)
I have many times thought of abandoning them - but I'm the only one with a boat that can motor against the floods. And I don't want the debris from their bodies jamming up the lock downstream.
Maybe next floods I'll just tow them over the land and abandon them there.