Labels are normally great things. Breakers and switches should all be labelled and labelled wires are really handy for troubleshooting. But when you see a label that says "turn off switch A before turning on switch B" you might have a disaster in the waiting. On one of our jobs, the owner had a shore power and inverter breaker side by side with the outputs combined, and below it a label saying "turn off shore power breaker before turning on inverter breaker." However, there was no physical barrier preventing them from both being on at the same time and both attempting to feed his boat at the same time. Nothing good can come from that, so it is important to be careful.
Sure, if the owner always follows the directions nothing will go wrong, but what if someone else is on the boat and doesn't know how it works? Or a kid comes on board and starts pressing things for fun? In this case, directions are simply not good enough and physical restrictions are required. A device is needed that will only allow one to be on at a time, such as a selector knob or sliders that block one breaker from turning on if the other is on. Not having a proper power transfer setup could not only cause problems on the boat, but also on the dock and beyond. What would happen in this case if the city power went out and you flipped on the inverter breaker (or a generator) and left the shore power breaker on? You'd be feeding power up the dock and risking the lives of hydro workers trying to fix the problem. Remember, a label can't compensate for an unsafe system.