There are many manufacturers of composites, many variations by each manufacturer. Mold is a bigger issue for some than for others. As a professional deck builder, I recommend checking with some of your local deck contractors to see what's available locally and has held up. Stainless steel is fine...just be aware that it is soft, will require pre-drilling in composites and when the screws break off will be a real pain to get out and replace. The pressure treatment in the framing will be more corrosive than the diluted salt water from the pool. There are quality coated "trim head" screws available that will do a fine job without leaving a large mark. Frequently with the trim heads simply tapping the excess from the screw hole with a hammer (NOT pounding it!) will push the composite back over the head and leave a hidden fastener appearance. Ipe is great as well, and is actually cheaper here than most of the quality composites. It also requires pre-drilling unless you use one of the specialty screws designed for hardwoods. As I mentioned in an earlier deck question be sure to follow all permitting requirements. Homeowners' insurers will usually refuse any claim for liabilities that occur on, around, or associated with any construction that does not have a certificate of occupancy. Pools are even worse.
But no other screws will last as long as the decking.