I know its too late now, but for future reference, get rid of all the rust and just use an epoxy
barrier coat like Interlux
2000 and then some epoxy filler. Obviously, get the boat
hauled out ASAP and get the problem fixed by a good welder. Dont be tempted to weld one plate over the other. The area has to be cut out, a new plate put in the cut out area and then the new plate needs to be welded on both sides.