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.