Re Flexseal - I want to hate it because it is one of those late night TV items. But I ended up trying it on my roof three yrs ago to patch a potential leak. City rubber roof, happily tilted not flat. I had a tiny leak that happened one time when the tail end of a hurricane came thru with a good bit of oomph left. No other time even with a snow load so I knew it was a pretty tiny opportunity. I figured if I got something on it before it turned major I could stave off the big job. I went up with both the spray and a roll of patch material and used both generously while I was there.
Not a drop since then. And there were a few places that were vulnerable to beginning leaks as time went on, where the seal on the edges of a piece of rubber was looking a little dry.
It is heavy so probably not the best plan for a boat restoration. But the stuff does work.