Have built several docks using PVC filled with concrete. Using 4" pvc schd 40 pipe, we drive the pipe until we hit hard pan (here in FL it is usually 4'. We wash the pipe in using 3/4" pvc attached to a hose washing on the outside or we use a 1 man post hole auger to dig in 3' or so. We then seat the pipe in the hard pan using a drive mechanism (the steel weight used for table umbrellas works great. Turn it upside down with the pole holder fed into the pipe)
We then cut the pipe to height and set the beams(or ledgers) that the joists will set on with galvy carriage bolts. We slide in one 5/8 rebar and pour in concrete. We install post every 8'.
My 1st dock done this way was installed 9 years ago in a tidal area. 5' x 50' with a 16x20 end. Is still standing with no deterioration on the sub structure. Has been extremely stable with no cross bracing. No settling, sagging, swaying...still.
The question of pvc vs wood post is all about durability...and pvc wins hands down.
