Thanks Todd and Paul
It is a carvel planked boat and looks pretty tight for being inside for 20 or so years. After looking at the youtube videos of caulking and reading up on the 3m products and sikaflex and boatlife it seems there is a lot of choice. I guess I'd be leaning toward advising the owner to go with the less permanent 3m 4000, sickaflex or boatlife. This way he can keep the boat on the trailer, ready to use without worry of leaking; and he can know that if there is a problem, the next repair man will be able to get at any problems.
If I get the job I believe I'll strip the exterior paint as much is chipped down to the cedar. Then remove the old caulking, sand the joint edges, then apply the caulking. If this is okay so far, the next question is whether to fill up the gaps with the caulking or leave an indentation for expansion. What do you think?