In addition to what the gentleman from the great state of Michigan says, heating the screwdriver tip also melts the old paint and varnish so the driver can get maximum engagement.
If you need to drill it out, start with a small drill that actually fits into the slot. Do your best to drill a pilot hole in the center of the slot and perpendicular to the shank. Then go larger to a drill, about the diameter of the shank. With luck and practice you should have drilled the head off. The screw, still imbedded in the keel, should pull through the hull. That will leave you a headless screw that you can get vise grip pliers on to turn out.
This knowledge is solely the result of working on @&$?ing Shell Lake canoes and boats......
Good luck...!