Stainless Steel Self Tapping Screw
These are A4 (316 grade) stainless steel screws which will resist salt water corrosion so long as they are not starved of oxygen. They are therefore not suited for work below the waterline in waterlogged timber where Silicon Bronze is a preferable material.
Arthur Beale says:
Self-tapping screws will often grip timber better than wood screws because of their coarse thread. Adding a little Lanoguard grease will help stop water penetration.