On the other hand, assuming a styrene hull, having prepared the mating surfaces so that they are going to be a snug fit, and having tacked the two halves together with dabs of cyano to ensure that they are correctly aligned, just run a fillet of styrene cement from the tube down the seam from inside the hull and let capilliary action draw it in. Then apply a modest pressure by gently clamping. The styrene cement works by dissolving the styrene that its in contact with. The two surfaces, having melted and being in contact, when the solvent evaporates, form a welded join. When its dry, a second run should fill any gaps left from the first time.
My ten year old Snowberry was done this way, has had plenty of mis-handling including ramming the concrete bank a few times, and the four hull parts are still solidly together. Since the join is effectively the same plastic as the rest of the hull, it was, and remains, just as waterproof as the rest of the hull. It also has the same coefficient of expansion, so changes in temperature are less likely to stress the join.