We recently had to trouble shoot a problem on a boat that had negative battery switching instead of positive. In North America, everything is switched (and fused) on the positive side of the battery. In Europe, some boats have both positive and negative battery switching (i.e. Beneteau) however, we have never seen a negative only battery switching before.
The boat had a new charger added (following ABYC standards), but since the existing wiring wasn’t following current standards it created new issues. Remember, current will always find a path. And since so many things are connected to the common ground on the boat, it is possible that many paths exist. And switching off one may just allow another to be found.
After the new charger was installed. When the negative battery switch was turned off, the main DC panel still had power, but only 8V and would not handle any loads. The voltage drop was due it to finding another path that was very poor.
During trouble shooting I traced the power flow: from the negative battery terminal, up the charger chassis ground cable (which the older charger probably did not have so this was a new path), through the charger AC ground cable, out the boat AC ground cable on the shore plug (going through a galvanic isolator), onto land, into the water, back into the boat via the bonding system, and getting to the main DC ground bus to the DC panel. When the boat was unplugged from AC Shore power, the DC panel lost its power. This current is really, really bad. This is stray current corrosion, and I was reading 0.8 amps flowing out the boat with everything on the panel off. This boat was also missing the required DC to AC ground bond. If this cable was there, there would also have been power going to the DC panel with the switch off going through this cable instead of the water, and would also have to have been trouble-shooted after adding the new charger.
It is interesting as my understanding was Beneteau and others added the additional ground switch (with the positive one) to prevent any stray current issues. But in this case, having only a negative one actually created a corrosion problem. Personally I do not like negative switches, and often they are no longer wired in the correct place anyways as people end up wiring things on both sides of it.