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Problem with supply side connections
Problem with supply side cables
Problem with user side connections (house circuit shorted)
Problem with house side cables (house circuit shorted)
Problem with breaker
Problem with battery selector switch (although only live should go through this switch, you should check to confirm).
Problem with battery
Put the battery last, as I think a ground fault or short on battery itself is not common at all. A dead battery, yes, common. But a circuit fault, I'd guess not common. Especially if the battery visually looks okay.
Check the battery cables and connections first.
After that I'd pop open everyone of your house circuits and try and turn the battery switch on. Then close one circuit at a time until the battery switch goes, and you will isolate the problem.
After that I'd test the battery (because it's easier than a new breaker or switch).
After that put battery 2 cables onto battery 1 with 12V circuits all open (off). If it doesn't pop breaker, probably a bad battery 1 breaker.
Just suggestions, although I'm not a marine electrician, so use my advice only with a high dose of caution and consideration, as I could be wrong.
Boat Name: King Kong
"Boat + Water = Fun"