Do I have to get off the ship at every port of call?

Unless specifically requested by the ship, for technical/legal reasons, then no, you do not need to get off at every port of call.

You are welcome to stay onboard and use the ship's facilities. Indeed, often the ship will seem less busy when in that day’s cruise port and may run special offers on services such as the spa.

Due to customs regulations, the casino and shops may be shut while the ship is in port.