The only PT wood in the water would be an underwater stabilizer between two arms of a 'U' dock. All framing is mounted on top of the floatation. Now the code mandates plastic (ABS I think) covered floatation. The oldest docks have an orange styrofoam, them blue was mandated, now plastic covered (for any new construction).

Dave

"In theory, practice and theory are the same. In practice they're not."