Most common shore power Amp&Voltage connection?

[mrdeepseafisher]

My boat has an existing 30Amp single phase shore power connection. However the AC has a separate shore power connection for that only. I am currently reworking the electrical system and was thinking about upgrading to a 50 Amp connection. I reviewed my intercoastal waterway guide to see what the most common type of connection was and it seemed that there were quite a few marinas with 50Amp connections. However, I could not determine if it was a 50Amp single phase 120V connection or 50Amp single phase 240V. My Boatowners Illustrated Handbook of Wiring shows both types but makes no recommendation. Any thoughts?