What are you running at the dock? Why the indecision about 30 or 50 amps?

For 30 amps, you'll need #10 conductors, #8 for 50 amps.

A 230v 30 amp circuit would give you two 120v 30 circuits at the dock. Are you running any 230v equipment?

Burying Cable

My current boat takes two 30 amp feeds, one for the boat and another for the AC. Most of the newer boats 40 foot and up take a 220/250 amp service. So if I run two opposite phase 120 lines I can use it now for this boat and if I go to another boat I can buy pigtails that turn these into a 220v system. If I run the 50 amp 220v line then I need pigtails (Y connector) to split the current to two 110 lines which I already have. I appreciate the link. I was really hoping that running 110v lines would eliminate the need to dig two feet. The water table is high and going down two feet may put me in water at high tide.

You can go 6" deep if you use rigid or intermediate metal conduit. The 12 inch exception for GFCI-protected circuits is limited to 20A/120V.

thanks

So could I run two feet deep for part of the way and then go 6inch with metal conduit?

Yes provided you're using the appropriate conductors for each segment and make the junctions in an approved manner.