I agree with Barefoot_Johnny. Stainless Phillips #12 panhead sheetmetal screws are not un-attractive. They really almost disappear, especially if you are using a rubber-backed, marine carpet. Even in Menards' Status brand carpets, the screws are minimally noticeable. And, yes, you may need to pull the plywood up in the future. And if you screw it down and then glue the carpet over the top, you'll never get the glue out of the Phillips slots to un-screw ALL those screws. You would have to grind off the heads, pry up all the plywood, and then turn out the screw shafts with a vice grips. So, ultimately, exposed screw heads is a small price. If you measure carefully and put screws at consistent intervals, the "pattern" it makes will not be ugly, in my opinion.
Now, in many fiberglass boats, you will see really shoddy craftsmanship, especially those from the mid 80's up until about 2000. The wood stringers are glassed to the hull, the plywood is stapled, let me say that again, STAPLED, to the stringers, and then one big sheet of carpet is glued to the floor. NOW that is a mess to fix when, NOT if, but WHEN the floor rots, because unless the boat is never used, even those stored inside will eventually get a rotten floor. OR it could be like the crappy way Lund Boats do their floors, the floors are riveted to the sub-structure so you are forced to grind off each and every one of those rivet heads.