You can start by placing a NEMA 4X enclosure at a convenient spot and place a sub-panel in it. You may want a weatherproof disconnect near the dock. I don't know if it's required. Seems like it should be.
A small pad (optional) and a few 4 x 4's and some galvanized Unistrut and your set if you want to make a way to mount it.
If the dock is ever hit by anything like storms etc, then replacing from that subpanel is all you need to do.
A company that has a lot of pedestals is
Midwest Electric Products, Inc., so you can check out their site.
Instructions for reading the NEC code online is in the sticky in this section.
In hindsight, a separate conduit for low voltage might have been useful too.
You probably want to feed the subpanel with a GFCI breaker as well.
You need #4 with a #6 grounding conductor for a 2.6% drop at 240 V at 50 A non-continuous.
You need #1 with a #4 grounding conductor for a 2.6% drop at 120 V at 50 A.
Actual loads and distribution in the panel can better be used to accurately size the conductors based on the neutral current expected. The neutral current is the difference flowing in each leg.