I know this is gonna get blasted but, the "infomercial dude" was on just this morning with a "spray on" (black in color) sealent that is supposed to fix leaks. The reason I thought of this particular post you made chuck is that, he sprayed down a screen door with this stuff and then used it as a "bottom" of a boat. The ad, you know the fine print stuff, claims he sat in it for an hour and never took on water. Now, I have no idea how this would work on a boat, but it IS made for gutters too so I would have to think it could handle the "wind factor". I just don't know about the constant punishment a boat gets going across the water. Just an idea, heck I have bought a couple of those things he advertises and they have suprisingly done exactally what he claimed they would (my favorite being that "puddy" stuff that comes in a clear tube and is STRONG AS HECK).
I have been watching to see if anyone has ever really thought to do this/tried it before, cause I honestly never would have thought to do it. Now I am curious if it would work or not for an older water craft I traded for. Heck, if noone answers maybe I will try it on this thing I got just to see if it works and let you know..lol