I did my dock electrical myself, and it was an inspected job. There are rules about min height above mean high water for recepts.
Ground wires need to be Green, not bare, like pool wiring demands.
You have a choice to do a sub, or not; with only the one recept. I did. But then, I ran 50A/240V to power a lift also.
It is a good idea, but I don't think mandatory in the previous codes to have that 30A GFCI protected. It is ABYC to deal with that in the boat.
Consider that a lamp, or a 15A recept, or anything else will demand that sub.
Also, I did a waterproof isolator switch just before the 30A twist lock. It may be code. Most folks should have it OFF, before shore power connection. and, the DIY El forum is prolly better than this one for questions like this. Lots of sparky talent there.
One more tip; minimize the flex conduit. It molds pretty fast, while the rigid pvc still looks good.
Lastly: I don't believe that a DOCK is considered a separate structure, demanding a local ground rod. I actually did a boathouse (with roof), and did local grounding and bonding including a lightning protection system. There are bonding requirements if this goes to additional structure, such as boat lifts, and even permanent water ladders. And, there is a minimum spacing from ladders to electrical controls/recepts.
