If your looking for an all Natural Wood, any Hardwood should do fine, more specifically Ipe. Ipe is widely use for Docks and Boardwalks (Atlantic City boardwalk for example) and has been known to last a good 20-30 years and running. As for sealing Ipe (or any Hardwood for that matter) be sure to use a product specifically Design for it, I've had success with Messmers UV Plus for Hardwoods and the Woodrich line up.

Also, be sure to use stainless steal screws!

I should have stated that the deck material is redwood.
I can afford that but want it to last another 15+ years.
ipe is out of my price range.
All fasteners will be #10x3 316 SS square drive.
I will pre drill and countesink all 1200+ of them.
I'm looking for a wood protector than will withstand my pools salt water. The salt concentration is compared to that of a human tear.
The deck has to be replaced due to failure of the fasteners, all my fault.

Maybe use Mahogany, better than softwoods and cheaper that Ipe. But if you use Mahogany buy one of the better types like Meranti or the like from a knowledgeable supplier like a real lumberyard so your sure you end up with a quality deck board.