I think what he meant was on a small boat you may only have enough room for two batteries. With two 12's on an A/B switch, when one falls off you can switch to the other.
With two 6 volts taking up that same space, you really only have one 12 volt battery, and when its gone, its gone.
Common sense will dictate your needs. If you have the space, and need that much capacity, a bank of golf car batteries can be a dream. If you dont have the space, or only use the boat as a day sailer, you wont need that kind of capacity. Two 12 volt marine batteries should easily be able to run nav lights and small intermittent loads for a weekend.