I've done "blind fishing" like this before. Go to home depot and get some Mason's line (string), a 1/4" or 5/16" flat washer, and a package of rare earth magnets (the little silver ones). Back at the boat, drill your holes (up to 1" dia should be ok), tie the washer on the end of the string and use the magnets to draw it through the pan and out. If the washer drops off the magnet (because the underside of the pan is a rough, unfinished surface) just pull it back with the string and try it again. Once the string is through, use it to pull your instrument wiring through. Leave the string in place, trimmed and tied off at either end so it's available if needed in the future.
Bob's your uncle.