I put a drop-skeg in a skin-on-frame a few years ago. As I recall, I went to the local box store, bought three or four different tubes of polyurethane caulk/adhesive and tested to see which was best for the job. I don't remember the name of the one I used, but I do remember that it weighed more than the rest.

Once sealed, I also tested to make sure it was watertight by blowing air into it -- and I did have to go back and seal a few more obscure leaks. The hard part, really, was reaching in to work on it.