adtech is great and should be really good for below the waterline. is this a boat that is being kept for long periods in the water? when a hull bottom is media blasted, most yards fix all the bad areas and blisters with 3m marine blister repair material. its a filler but meant for below the waterline and its rock hard. that has always been the repair material of choice. neither adtech or 3m is meant to repair actual glass cracks though. if the fiberglass itself is cracked fairly deep then that needs to be a structural repair done with glass and resin then you can skim with adtech or 3m. keep in mind you cant paint a hull bottom that stays in the water with regular paint or single stage. you need to roll on a coat of an epoxy barrier coat then antifouling paint. regular poly will blister