Are you talking of a deck cleat? Those are installed on the overhang so you can bolt them from inside the cockpit tub. You do not want to pierce the hull unless you put in an inspection port so you can put a backing plate.
All hardware should have a backing plate even if it's only a washer. It should be caulked to prevent leaking if that's desirable. There are numerous methods to accomplish this also depending on location and hardware.
Sunfish have backing plates under their existing hardware. If your "new hardware" will be expected to take a heavy load, a backing plate should be added. Wood, Trex, plastic cutting board, aluminum or stainless steel are alternate backing plate materials.
Inspection ports are the preferred access; however, cutting through the bottom or sides are options--to be resealed afterwards using a fiberglass repair kit--$30.