“I’ve wondered for some time why my boat has a switch at the main battery panel that turns the DC negative on and off. I’ve sent in a photo of my main battery switch arrangement. None of my friend’s boats have this feature and I’m just curious about it.”
Switched DC negative is typically a European electrical feature that you rarely if ever see on American built boats any more. In commercial applications where the hull material is steel or aluminum, we used to see this approach taken quite often. Furthermore, you may have actually seen double-pole circuit breakers for the DC branch circuits that would simultaneously open both the DC positive and negative conductors. The origin for all of this was a valid fear of stray electrical current causing rampant electrolytic (stray) current corrosion. By completely isolating the battery(s) a boat owner could pretty much be assured that without any power source, all chance of this happening were eliminated.
Today however, I consider this approach as a bit of overkill as long as the boat in question is wired properly, which explains why you don’t see this approach taken on most boats today, regardless of hull material used.