Tom, there are 3 levels of pressure treated wood. These are "treated to rejection", .40 retention, .60 retention and they are ranked from the lowest amount of CCA to the highest. Care to guess which one you get if you buy "landscape timbers" at your local yard? .60 is the only one that stands much of a chance surviving a year around water environment.
The real difficult part of the question is that the treaters are changing the chemical that they will be using this year. While the new formulations are supposed to have a comparable resistance, based on use in Europe for some years, they just have no track record here. Personally, I would do what I had to do to get some of the 60, if I was in your situation. But I still would not hazard a guess as to how long it will last. Too many things can happen.