Others may differ on this, but IMHO - inverters are not of much use on a boat assuming you have marine-grade equipment installed. Inverters take a lot of 12V juice to power 120V accessories. You usually need at least one 8D battery connected to the inverter if you want any sort of life under a reasonable load. 8D batts weigh about 125 lbs - so you are adding a lot of weight there! A small genset might be a better choice.
So when would you use the inverter? Got a frig? Most marine frig units are dual-power and you can run them off your 12v system while the engines are running, then turn it off so you don't drain the battery. The frig on my cruiser will stay cold for 2 days without power, so I just keep it on the 120 volt source because I know I will run the generator at least once a day when we are out on the water. What about 120V lights? Again, most boats have 12 volt lights, and you can run those as long as you want if you change the bulbs to LEDs. Television? TV's are easy to find that run on 12 VDC - in fact the circuit boards on many LED/LCD televisions are 12 volt based. The big deal would be air conditioning - but an inverter that would run an A/C unit (think 10-15 amps at 120V here) would cost $2,000-$3,000 and require some monster battery source to run for very long. Again, here is where a genset makes sense. You can run a 12,000-16,000 BTU air conditioning unit with a 2,000 watt generator if you get something like the SmartStart that reduces startup load.
The modern generators like the Honda 2000 are very popular, well-made, and quiet, but they are not water cooled or designed for enclosed use like a marine generator. You would need to have one of the Honda generators located on a swim platform or somewhere in open air. A marine generator can be installed in a compartment and permanently wired into the boat's electrical system. But there is BIG difference in the cost of a Honda and a marine generator! Figure $7,500-$10,000 for a good marine generator (4-7 KW) installed with wiring and controls.
One more option (not real popular) is a 120 VAC alternator for one of the main engines. In this case, you get your AC current off a second alternator on one of the mains that is wired to produce 120 VAC. Some small RVs use these when they don't have room for a generator or a big battery bank. Not a bad choice, but you have to keep the engine running to get your AC current. Also the alternator needs to be marine rated for use in the engine compartment and those are hard to find and somewhat expensive.
I had a 1500W inverter on a previous boat but almost never used it. It would not run the A/C, and used a lot of battery power to run the frig. About the only time we used the inverter was to run my wife's 900W hair dryer - and I usually ran one of the mains when using the inverter to keep the battery charged - that seems to defeat the purpose. Maybe a 12V hair dryer would be a better choice?
So you need to decide what your needs are and what option best fits them. The inverter route seems only to work for some specific, low-current application that requires 120 VAC and can't run on 12V. I was thinking laptop, but then realized it's easy to find 12 VDC power supplies for laptops. Maybe a home-type flat screen TV? In this unusual case, a small portable inverter you plug into a 12V outlet might be all you need.
