Re: SOS please help!! '72 Grampian 30'
A rasp, typically a wood rasp, is not meant for hard materials like metal or fiberglass. A four-inch or adjustable hole saw could run $45 in the hardware store but it will make many more holes, much faster.
Avoiding the ever-present temptation to "make do" or "cheap out" usually makes the jobs go faster, and cheaper in the long run.
Spray foam generally doesn't have compressive strength, which your deck core needs. And if you've got some soaked core here and there? Drill some 1/4" holes in a checkerboard pattern every foot out from the soaked area--on the underside if you want to hide them, but don't mind dripping IN the boat. Otherwise, on deck, and you'll have to reseal them before it rains. Odds are that there's a much larger area that really needs repair. If you don't get it thoroughly dried and sealed, it will get worse. (Or hire pro to tap and meter, if you don't want to make holes and refill them.)