I think an AC selector switch that is built to go directly from generator to ground power is a bad idea. Unless you are very vigilant and make sure to turn off all loads prior to moving the switch.
Our boat has an old Onan 7.5kw generator and the selector switch is an as-supplied part from Onan. It is a four position rotary switch. The two positions on either side of 12 o'clock select ground power or generator power. A little indicator light comes on to show there is power but the switch is not connected to any AC loads on the boat. The next position down from either of the upper positions connects the switch to the AC breakers
However we never change from ground to generator power without first turning off the master breaker on the AC panel as well as all the other breakers. We make the change, then turn the master breaker on, and then the breakers on for the devices we want-- hot water heater, refrigerator, etc.