Installing a sub panel on a fixed dock (boathouse)

I live in NE Alabama and am building a boathouse. Part of the dock is built on 6″ I-beams driven into the lake bed and has wood decking on top, the boathouse is being built on wooden pilings and it is attached to the dock with I-beams. I plan to install a 60 amp GFIC at the service box for the home, then run 3, #4 THWN wires and a #8 ground in pvc approx 160′ to a sub panel located on the boathouse and use regular breakers there (I will have 6 or less breakers). I will keep the ground and neutral separate at the sub panel. After pouring over the NEC and reading about every article I can find, I still have a few questions.
I understand the height location requirements of the sub-panel, but can a weather-proof sub panel be located on one of the supports for the roof (on the fixed dock itself? (there will not be a wall, just a roof).
Most of the info I can find about Equipotential Plane is for outdoor pools or spas. Do I just bond the I-beams together then drive a ground rod (3/4″ water pipe no less that 8′ long) into the lake bed, next to an I-beam and run a #8 copper ground from the bonded I-beams and ground rod to the sub panel ground?
The NEC talks about having the Equipotential Plane extend 36″ around the disconnect, which I believe in my case, would be the sub-panel?
Does that mean that there should be an Equipotential Plane (wire grid) on or beneath the wooden deck directly under the sub-panel?
Thanks for your expertise
greg