Hi Nigel, I would expect a good seal with some sikaflex 291i (or equivalent), if there is a lot of play in the bolt (and if the bolt length allows) a backing pad glued in position should help. Depending on the state of the hole filling with epoxy and re-drilling might be required. If the nut does not have a metal washer – then add one.
If you can post a picture showing the problem that would be useful. (Just copy and paste).
The GRP boats should have some kind of permanent buoyancy to stop them sinking if the buoyancy tanks fail. But this would have varied over the years. A block of polystyrene moulded into the aft tank was indeed a normal option, usually with some under the side decks too. There is quite a good discussion here with details of what is required;
hope that helps