My boat has dual 30amp 120v. For several years I had run a 50 amp (240v/dual-120v) into a 'Y' splitter from the dockside 240v/50amp => (2) 120v/30 amp. At the time I considered asking the marina to downgrade the pedestal circuit breaker to 240v/30amp for better
safety. Never did. I also have a pair isolation transformers on the boat, and the shore
power connection once on the boat goes cord inlet=>~3feet=>breakers=>transformers=>~10feet=>ma in breakers on the panel.
I would however recommend if you do this, and if it is your boat's regular slip, to have the marina simply downsize the breaker on their side.
Another interesting part of all this, is that for the marina I was at, doing this was fine, as long as the 'Y' splitter was on my boat, and not dockside. I guess for the
insurance companies, if my boat catches on fire and burns down the marina, somehow that is okay as long as it wasn't the marina's fault? CYA and all that? Who cares about overall
safety as long as it is somebody else's fault?
So yeah - anytime you use an adapter to 'downsize' an
electrical circuit, you want the circuit breaker upstream of that downsized to match. My understanding anyway. Should be a
cheap one time fee for them to do that, and you will probably make good friends with the marina owners by asking and doing it right.