I'm planning to run about 200' of 12/2 UF-B buriable romex from my barn (which is powered by solar) to my dock.

12/2 UF-B is $109 per 250ft roll at my local Lowes; 10/2 is $188 per and for 500' the 10/2 might be better from a voltage drop perspective.

I'm planning to rent a 18" hydraulic trencher from Home Depot ($95 for 4 hours) to dig a trench for the cable.