Re: Deck scews, can I use coated deckmate screws?
I used drywall screws to mate my transom boards as well as my stringer sections. As F14 said, as long as you seal them up, they won't rust. I also used deck screws to couple my deck to the stringers. I just dripped poly into the holes I predrilled and screwed them in before it set. They are now encased in poly surrounded by fiberglassed wood. If water hits the screws, I will be way more worried about the wood than a few screws. I say, don't stress about the screws and get the wood protected instead. After all, the early space shuttle capsule walls were only as thick as tinfoil and those guys survived space. Not everything must be awesome to work.
That said, if you can find SS screws for a decent price, all of this is moot.