As an answer to the question "why does Brody dredge the bottom?", he does it by accidet when trying to 'safely' pull the rope that the kids threw to him over to bring them into shore. Because he boat ran aground, the dredge only has to pull the cable up a metre or so to have it free of the water.
Its referanced earlier, when the police dredge it up in a tension mis-fire designed to throw the audience a curve ball, like what Spielberg did in Jaws 1 with the unannounced attack (musically silent). Not as good as in the first film though, but I suppose its the same kind of set up as the "You screw around with this stuff and it'll blow up!" (sic) said by Hooper in th first movie as a nod to the demise of the shark at the end. Think the quote in Jaws 2 is along the lines of "If we're not careful, the whole island will be blacked out!" or something.