There are plenty of coastal locations where the new composites made by Trex and other companies would make intelligent decking choices. But on Nantucket, where a traditional look is preferred (if not required by code), we use only wood.
Even if you're using the new synthetics for the deck boards themselves, you'll still have to build supporting posts, beams, joists, and ledger boards from wood. So everything we've said about structural stability and sealing still applies.
If you're using wood, don't consider anything that is not pressure-treated or naturally rot resistant. Pressure-treated yellow pine is the most common decking material. Natural, rot-resistant woods such as mahogany, Douglas fir, and Brazilian hardwoods are expensive. But many of our customers prefer them because they are the most natural looking and they weather beautifully with little or no care.
You should expect 20 years of use from a deck properly planned and built. And that's without applying special stains or treatments. Some people love to see water bead up on treated decks, but I'm not a fan of unnecessary maintenance. You're paying for pressure-treated stock or for woods that naturally resist decay, so why spend Saturday spraying your deck with an expensive coating you don't need? Take the money and play golf.