If you want to mount something solidly on the inside surface of the bilge/hull area, epoxy such as Marine-Tex or JB-Weld comes to mind. Also, you could 1st mount a block of wood that allows you to screw-mount items to. The block of wood could even be covered in fiberglass cloth and epoxy resin, to make a super-nice and strong mounting location great for any variety of marine electronics.
You might need a fool-proof way to hold it in place while epoxy is curing, I usually jamb or rest a piece of lumber against the block and use a bit of tape such as duct-tape as a dam to hold the epoxy in place and keep it from running down the side. To thicken epoxy if not viscus enough, I mix it with some sawdust to a peanut butter consistency.
Partial list:
1/2 to 1 quart of epoxy resin and hardener
2 sqft of fiberglass laminating cloth
1/2 coffee can of sawdust, maybe use about 1 cup
12"x10" seasoned/kiln dried wood block (not moist with water)
Nitrile rubber gloves
Mixing utensils
Plastic Spatula
Duct Tape
1 quart of acetone for pre-cleaning the surfaces and cleanup afterward
1 Rattle Can of white epoxy paint
