MAYBE.... use a torch to "can-open" the back ends of the pontoons, preferably AFTER locating a sufficient quantity of heavy-duty, sealable, cylindrical plastic jugs, buckets or drums (5-gal?, 15-gal, 30-gal?) that can snuggly stack end-to-end inside the pontoons. Coat the jugs' exteriors with beads of polyurethane expansion-foam before ram-rodding them; one behind the other, into the thoroughly dried pontoons. The poly should expand and "stick" to the pontoons' interiors - displacing even more water, and keeping the vessels from banging round inside the pontoons. Then, close the "lids" of the can-opened pontoons and tack-weld them into their original positions (without melting the last jug). After the spot-welds cool, poly-fill the non-welded portions of the seams to make them water-tight.
