You are in Article 555. I don't see anything that limits the height of conduits above the water, in fact the standard practice for boat lifts is to run the conduit under the water.
The limit is for the splices and devices,
All electrical connections shall be located at least
305 mm (12 in.) above the deck of a fixed pier but not
below the electrical datum plane
PVC seems to be the choice here but we are dealing with salt water, that looks like a lake and it is much more forgiving.
I would run the PVC under the dock. That is also pretty standard here. "T" the "UP"s for the lights, just be sure there are no splices below the dock.
