Page D-2 of my Westsail Service Manual describes the two different methods used by Westsail to install the deck on the hull. Anyone not having that page, which was updated in June of 2006, can email me and I will send a copy of that page.
The deck flange was either screwed or bolted to the hull flange, depending on when the use of bolts was started, which was after I left Westsail. The caprails were always attached with self-tapping screws.
Sealing the exterior seams, and also recaulking the hawse pipes and stanchions should be the first procedure made to stop the leaks. It is the easiest, and quite often works.
If this does not do it, then the caprail should be carefully removed, and the top of the deck to hull flange sealed with polysulphide or polyurethane caulking. It should be then water tested to make sure there are no leaks. The caprail can then be reinstalled, taking care to seal around each of the screws used to attach the caprail.