Many European boats that I have seen have a DPST master switch that breaks both positive and negative with a single
switch. The most common reason I have been given is lightning
protection, the negative side on many boats is frequently bonded to many things (engine block, thruhulls, maybe rigging
, etc.) and lightning
damage probably occurs more frequently through the negative side than the positive. Doesn't help anything (like most of our most expensive toys) that has other connections (antenna, signal wires, etc.), but for solely electrical equipment
breaking both sides does provide some protection, with the negative open as well your equipment
is no longer bonded to the boat's ground when the bolt hits.