Below the water line sealants

I recently purchased a 2016 Sun Dolphin, American 12 plastic jon boat for my granddaughter and I to float rivers and creeks. This boat has an inner, and outer hull. It's never been in the water and has set behind my friends barn since he bought it new in 2016.

Once I got it home, I noticed ants crawling around on it while in my garage.  Removed the thru hull fitting that goes through both hulls and killed all the ants. I heard water sloshing around and drained about a half gallon. There is another thru hull fitting that only goes through the outer hull. I can't get it out without separating the hulls.

I think the rain was seeping in around both fittings. Which marine grade, below the water line sealant would you recommend? I've got 3M 5200 here at home and have used it on my bass boat, but it is not recommended for plastics or rubber