If you google
"dockbox" you can see hundreds of examples.
I think what you're really wanting us to tell you is that a metal fridge is a dockbox. I think a metal fridge is a fridge. Whether a fridge can also be a dockbox, or vice versa, requires a level of semantics of which I am not capable.
At the end of the day though, the dock belongs to the marina, so they get to say what you can put on it.
If they say a fridge is not a dockbox, so be it.