Now you've got the hull sanded down, I'd fiberglass the outside. Done right it will be strong and leak proof.
First, buy the finest grade of glass cloth you can find in the hobby store; buy a small disposable paint roller; then buy fiberglass resin; at your auto supply store buy car body icing. Its a very fine body filler that is just like cake icing. Very easy to apply and sand.
Second; cut the cloth to fit the hull. If it overlaps the edges of the hull a little, thats not bad, it can be trimmed later.
Third; apply the cloth over the hull. It can be held down at the corners by a few drops of cyano.
Fourth; mix a quantity of resin, and starting at one end of the hull, pour it along the keel (I should mention, the hull should be upside down for this operation). Using the roller, roll the resin down over the hull to the beam ends. Do it gently and don't force it as the cloth will pull. Work from side to side and work quickly, but gently. The roller will leave a very smooth surface when done.
Fifth: When the resin has set, sand the hull with about an 80 grit paper, then sand with about 120 grit. Mix the icing as per instructions and apply a thins layer over the whole hull.
When the icing has set, sand again with 120 grit and prime the lot with scratch and fill primer. lightly sand the primer and check for low/high spots. Fill the low spots with small amounts of icing body filler. A final sanding and priming, then paint to taste.
Done this way, the hull won't leak.
John