Most retailers will sell stainless steel machine screws and not name the alloy. "Stainless Steel" encompasses a large variety of alloys, and you'll often find that an inferior, and cheaper to produce "Stainless Steel" is what's sold when the alloy goes unnamed. We only carry Type 316 Stainless Steel Machine Screws, and won't sell you anything less, as it's not what we would use on our boat.
316 (A4) Stainless Steel is far more corrosion resistant than it's 304, A2, or 18-8 Stainless Steel counterparts, and should be your go-to alloy in a marine stainless steel application.
These flat head 316SS machine screws come in imperial sizes, have a countersunk (CSK) head, and are available in either phillips or slotted drive.