What type of material are you fastening to? Is that aluminum or ply wood? Either way, self tappers won't hold up. The integrity of your waterproof compartment will depend on installation procedures, use lotsa high grade marine grade silicone caulk in any drilled holes and on your fasteners too!
If thats aluminum, I'd recommend stainless steel machine screws with loc-nuts and a reinforcing plate or plates on the underside. If it's plywood I'd use Stainless star nuts... and then there's always good ol' Construction Adhesive!