I found a product called Eternabond. Roof Leak Repair Products & Sealing Tape | EternaBond®
It is a popular fix for Motorhome owners with roof leaks at the seams. It was developed for the navy to seal underwater tanks and it can be applied underwater. It is a tape with a protective shield over it. Once you pull off the shield and place the tape you better have it where you want it because once it seats it is not coming off.
It would not be best used where it would get scraped and worn, so you might need to put it on the inside of the boat. I used this product on my RV and I can tell you it is permanent when you put it on.
SeaRay
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