I would stay away from anything in a rattle can, what is your painting budget? where will you be fishing with it? alot of guys have used truck bedliner coatings to help dampen sound. If you choose not to go with foam atleast have a nice big bilge pump. I would go with foam personally, however in a jon boat I would use styrofoam sheets instead since they won't retain any water. Use epoxy, especially if you will be in salt water at any time, 6oz glass is ok, but use atleast 3/8" ply. Coat both sides and especially the edges. Glass atleast the top, but I like to glass both sides, it will take some flex out of it. As far as the title goes, have the owner go with you to the courthouse, $7 gets you a reprinted title, otherwise it can be a headache.