Marine dock project — Type 316 fasteners corroding after 2 years?

This is concerning. I used 316 stainless steel screws on a dock project in coastal South Carolina two years ago. Client sent photos showing what looks like surface corrosion on several screw heads. These were supposedly marine-grade 316 from a reputable supplier. Has anyone else experienced this? Starting to wonder if I got mislabeled 305 or even 304.