Thanks for the various responses and opinions.
First of all, look at the
www.woodrx.com web site and you will see that they strongly state that their products do not create any environmental problems.
Next is the very legitimate question of why do anything if the wood is already pressure treated. My thinking is to extend the life even longer by preventing splits due to aging. Also, the wood is getting quite light in color and I'm thinking that a preservative with a stain would blend better with the existing dark stairs near the dock.
The company representative I spoke to at length was quite knowledgeable and helpful. Final decision not made yet due to other considerations but the fact that the products can be ordered through Home Depot with no shipping charge is real plus.