Bob,
In 1990 I built a house on a large pond and used 6" Cedar ship lap siding on it. I drove a 6' piece of this siding in the mud along the edge of the pond to track the water height. This same house had a large Cedar deck using 2x6's for the floor that were never treated with anything.
In 2000 I sold this house and built a new one 200 yds. away on the same pond. It has been 25 years now and I can still see the Cedar siding sticking up on the other side of the pond and the "neighbors" deck still looks as good as it did when built.
Randy