Originally Posted by Minnewaska
Not sure what you mean by not needed. If you mean that separate breakers are not absolutely necessary to safely protect a circuit, I agree.
However, in the event of smelling an electrical fire (the most common form of boat fire), wouldn't you want one master switch to kill it all? In addition, if you're up on the hard or away or just need to kill all parasitic loads, a master is very helpful. That may not be a breaker, but often goes through a kill switch of some kind.
A main breaker is not needed because the house bank should be fused within 7" of the positive post. That fuse will protect the wire to the main switch and from there to the main DC panel, where each circuit has a breaker or fuse which it must have. Whenever the wire size drops a new fuse is required if the main fuse is below the ampacity of the smaller wire.
As I posted a sub panel or fuse block is often used for "always on circuits" like the bilge pump or refrigeration. I also said a switch can be installed to isolate the sub panel or fuse block if desired. It is a good idea but I rarely see it.