waterproof

Be sure to use a caulk with high adhesion, flexibility and strength - I used a polyurethene caulk after receiving advice from Bill Layton. and found this to do an excellent job.  Don't use a silicone product whatever you do.

I've recently bought a laser and found that it leaked like a sieve along the hull deck joint.  Two gallons of water in the boat during a rainstorm!  This is a pretty poor design in my opinion,  only caulk and ss staples hold the two together.  Water runs off the deck and pools along the top of the rubrail with nowhere to go but into the boat.  I used a polyurethane caulk on the inside - great adhesion plus flexibility but impossible to remove.  I then fibreglassed the hull and deck together, which is probably the best and most permanent solution.  I had the galley out of the boat for a refit so could also do the fibreglass work here.

I had pulled the rub rail off the boat in sections and caulked it back on, but was not happy with the result - the caulk didn't seem to grip the rubrail well enough.

It seems to me that the worst area is at the widest part of the beam, and I surmise that if the boat ever hits the dock or is otherwise pranged at midships the hull deck joint flexes then springs open a bit and you've got a problem.