Looking at the water stains on the underside, it could indeed be Redwood - very similar to Western Red Cedar. Lots of natural oil to resist rot, but pretty soft wood - will mar easily. Not too big a job to pull the boards out, sand, and refinish. Many options available from simple oil to spar varnish to varathane type products. As DrewP mentioned, Mar-K did a wathering test on various finishes.
Great, so how do i know what type of wood is in there right now... short of tracking down a forester?
