How to fix a crack in my jet ski hull?

Get some fibergl cloth and fibergl resin. Mix the resin and put a thin coat on the crack. Than you put the cloth over the crack like a band aid and put more resin on the cloth. The cloth will soak it up and will eventually harden depending on how much hardener you added to the fibergl resin. You can repeat this step and sand it down to look good. Wear a mask because sanding this will kill your lungs. If this doesn't make sense look it up online its very simple.