What is it screwed into? Aluminum? Fiberglass? Or is it a bolt, with a nut under the deck?
One trick is to use a dremel to cut a slot into the head of the screw to accept a big flat blade screw driver.
With seized screws I have found an impact screwdriver often works. You hit it with a hammer, and it simultaneously digs into the screw and turns it. The combination of the two will break it loose more often than not.
If it is through bolted and you can access the nut on the underside, you are best off just grinding the nut off.