I cant remember the company but there is a product for riveted boat that you pour in on the inside and it seals . You can also put in new rivets and if your not going for looks get some stuff called 5200 made by 3m , once its on and dry its almost impossible to remove . We used it to seal the seams on some pontoons before and years later its still there . All else fails get it rhino lined , although it will cost more to do it that what the boat is worth .
roll in truck liner
fiberglass will work but it does not last long on aluminum surface that flexes like the johns do . If you ever have the option always go for the welded instead of riveted
Danger is real , Fear is a option
Omnem dimittite spem, o vos intrantes
You live more in 5 minutes on a bike like this going flat out than some people live in their lifetime.