Jeff,
Did you cross brace after the wood post were in the water? Did you pre-drill holes for bolts?
I could pre-drill a couple of 1/2inch holes in the 8" pvc pipe, then slide through a long galvanized bolt before I fill the pipe with concrete. Then I could later attach a treated 2x4 to for cross bracing.
I am not to concerned with cross bracing perpendicular to the bank. I think since my docks ramp will be tied to a concrete slab on the bank, I don't think that I will get much movement that way. But, I think eddie is right, it wouldn't hurt to have some cross bracing, especially parallel to the bank (between the front and back two piers).
I was also thinking that I could also drop a T post in the center of the pipe (along with some rebar) for added strength.
