i had a few rivites leaking on my boat and i used an item from cabelas that is made to fix the leaks around rivits and small holes it worked great cost about 6 bucks i have not had a problem with it at all the stuff flexes with the boat and can be painted
here is the link for it at cabelas
http://www.cabelas.com/cabelas/en/templates/links/link.jsp?id=0001390010655a&type=product&cmCat=search&returnPage=search-results1.jsp&QueryText=boat+patch&N=4887&Ntk=Products&Ntx=mode+matchall&Nty=1&Ntt=boat+patch&noImage=0
give it a shot worked great for me and can save big money on a welder