http://www.marinetex.com/products/ma...e-tex-flexset/
Specifically states that it can be used above and BELOW the waterline. I would spread it above the crack in the hull and below where there is water contact.
The fact that it's flexible is key as well, you don't want a hard epoxy to crack.
Oh, and it states that it will bond to Polyethylene which is key.
Then test water worthiness of craft.