For a main AC switch the hot and neutral are both opened. That way it there is a reverse polarity issue in the marina you can be assured that there are no hot wires. The individual circuits are single pole, opening the hot wire only on most boats. I say most because a friend has a French built Beneteau CC36 that uses double pole breakers for every AC circuit.
On a metal boat double pole switches are often used for all circuits, AC and DC, so when it is off it is guaranteed, for stray current reasons.
Living aboard in Victoria Harbour