On US docks, you will find "220V" at a 50A 240/120V outlet. Key things to note are:
1) It's going to be between 230V to 240V, not 220V. You will need to be sure your appliances are OK with the greater voltage.
2) As you know, it will be 60Hz, not 50Hz. You will need to confirm that's OK with your appliances too.
3) Although Dockhead is correct that there is 3-phase power in the US, what comes out of a dock plug
phase, either 30A@120V, or split-phase 50A@120V/240V.
4) 30A outlets are all single
phase 120V, not 240V
5) To pick up 240V, you will need to attach to a 50A outlet. You will need a 50A cord up to your boat breaker which can be 30A to protect the boat wiring
6) The 50A outlets are split phase 120V/240V (L1, N, L2). 240V is available between L1 and L2, and you don't need N unless you need 120V.
7) Some docks have 100A service
for mega yachts, but I'm not sure how it's set up. It might be 100A split phase, or it might be 3-phase. I don't know, but I doubt you would want to connect to that anyway.
Now you might now more, or you might be even more confused :-)