I have a friend, Dick Frost, who sings a version of this, but his version "doesn't have any ships in it because Bucks doesn't have a coast". On the other hand, I think the answer might be "deep", as a heavy anchor dropped too deep would possibly cause the vessel to keel over. Funny idea, though, when the object of the song is to warn someone not to enter an embarassing situation - the bit about ships in this version does not seem to suit the concept.