The good thing is that if you buy a quality generator, and install it properly it will be a one time purchase.
I do not consider a generator to be "optional" on a over about 35' boat (depending on the configuration of course), but thats just me.
This is true for boats with or without AC. People talk about light loading, but that is really a problem with their load management during generator run time.
Here's a good example...
With no other loads and a good size house bank and charger you can and will load your generator well above any point of "damage" with just the battery charger. Our battery charger is 150 amps. We set the maximum AC input to be 25 amps, and the charger sits at something over 20 amps during much of the recharge cycle.
The charger provides (all by itself) enough load on our 9KW generators engine to not cause any issues.
Add that to the 10 amps it takes to run the hot water heater and we are pulling 30 amps.
Choose to run the watermaker at the same time adds another 10 amps.
Wash some clothes and there is another 10 amp load (abet intermittantly)
Cook some food and you're drawing another 10 amps.
It's easy to see that properly managing your loads will keep plenty of load on a moderatly sized generator.