If I was going to use composite decking I would use Latitudes Marine Composite Decking.
http://www.ufpi.com/product/latitude...ing/marine.htm
I have their standard (non marine) decking at my camp and it is extremely durable. I have a couple pieces I have submerged under water (used as a shim) for over a year now and no issues of swelling or anything at all. Harvey Industries in Manchester sells it. I am very impressed with it.
If I was going to use wood, I would use Ipe only. This stuff is like iron!!
FWIW;
Dan