Cast epoxy alone is not a good repair. Epoxy is an adhesive not a casting material. If you do use a casting method the epoxy should have high a strength filler added to it. Adding cut up fiberglass mat is one solution. I would research West epoxy's product guides. If you do cast a plug in place the area has to be as clean and DRY as possible. The boat has to be hauled to do this repair. After sanding and vacuuming the area direct the heat from a ceramic space heater on it to drive out the moisture.

But as others have said: The yard broke it - make them fix it and fix it right.