It’s rare I’ll make an unqualified statement like this, but: 3M 5200 is the best boat goop ever, period. In fact no other sealant/adhesive even comes close.
This stuff can be used above the waterline, below the waterline, and in any part of your boat. If you use it right, it’ll form a bond between fiberglass so rugged that the fiberglass itself will break before the 5200 gives up its grip. And it stays this way for years on end.
So, what exactly do I mean by “if you use it right?” There are a few things that can weaken 5200’s ability to make that strong-as-steel bond. One is allowing it to get wet before it’s fully cured. And in this case, the cure time is rather extensive at seven days. But the importance here can’t be overstated. True story: a guy I knew pulled his boat, replaced the through-hull transducer, sealed the whole affair up with 5200, then re-lunched the boat an hour later. Within minutes the 5200 washed away (of course) and the boat began to sink. Luckily, still being right at the haul-out, he quickly pulled the boat. The next day he re-applied the 5200, and this time, allowed it to set overnight. The next day he launched the boat, and idled it across the harbor to his slip. Where it promptly sank. Yes, it does say in the directions, right on the tube, to allow 5200 to set for seven days—and you really do need to do so.
Another problem 5200 users will encounter is dirty or oily surfaces. If either is on one is present on the fiberglass you’re trying to patch or leak-proof, you need to do some prep work before applying the super-goop because contaminants will greatly reduce the chemical bonds it creates.
Some people hate 5200, because they say they can’t get rid of the stuff later on, once it’s hardened. This used to be a real problem, but not anymore. A product appropriately called Anti-Bond dissolves 5200, making it easily removable with a rag or Popsicle stick.
Bonus Factor: 5200 isn’t just great goop for boats, it’s also excellent for a countless number of fix-it jobs. Re-attaching the soles on a pair of forlorn shoes; affixing a piece of trim back on your car; putting that rod butt back in place once and for all—pretty much everything and anything that needs to be stuck, glued, plugged, or gooped can be tackled with 3M 5200. I love this stuff!