Splash Zone is a two-part epoxy. When combined, the parts create a wet paste or putty. In my opinion, it should have no problem plugging up that hole. This is precisely what it's made to do. Please research as appropriate. Apply from beneath using a long skinny implement (a paint stirring stick cut in half the long way, a discarded batten, a piece of wooden moulding, something like that). Allow to dry for 4 hours. Do a second coat. Maybe do a third coat. But don't just goop it on because you can't reach up into the centerboard enclosure with a sander and this stuff dries seriously rock hard. Wear gloves and goggles, especially since you're underneath the application spot.
I had a leak where the mast support pole attaches to the top of the swingkeel trunk (under the black plastic covers). I actually suspect the hole was always there from the time of manufacture, but leaked only with rough waves, substantial healing or fast motoring. A single dollop of Splash Zone repaired the problem immediately. I'm totally comfortable working with fiberglass, but I think Splash Zone adheres better, doesn't drip or run, and can be applied to difficult to reach locations.
Other than coming out of the water for 3 days to be painted, my 26x has been in the water continuously for at least 2 years. No leaks.
. As your hole is below waterline and into the ballast tank something like the repair in this video would be better than putty imho.
. But I would probably go to a known source of expertise on it. You probably aren't enjoying this enough to want to do it again.
