Prepare yourself with an on-board emergency toolkit that includes some type of epoxy repair putty that can be mixed, applied, and cured under water—which becomes especially important if you're off shore and unable to haul out.
Two examples of underwater repair putties are Pettit Splash Zone Epoxy and TotalBoat Underwater Repair Epoxy. These two-component epoxy putties fill and seal leaks in fiberglass, wood, aluminum, and steel hulls.