These hints are from Jerry Olson for the annual lighted boat parade PYC puts on around the 4th of July each summer:
Lighting up your boat:
Assuming that you find some miniature lights, or you dig out the Christmas decorations, the next problem is power. These days, small power inverters (that convert 12 volts DC to 120 volts AC) are common and inexpensive, but what size do you need? The answer of course depends upon how many lights you are putting up.
Miniature light strings come in sets that are multiples of 50 lights. That comes from dividing 120 volts by 2.5 volts per light (the real answer is 48 but the manufacturers rounded it to 50). Each group of 50 lights uses 25 watts of power, so a 150 light string uses 75 watts. Six strings of 150 lights uses 450 watts. That tells you what power of inverter you need.
Next you need to connect it to your 12 volt power system correctly. Let’s assume that your inverter is 100% efficient (most inverters do not use much power for itself). To figure out how much current you will be drawing, divide the wattage by 12 volts. For our example of six sets of 150 lights, that would be 450 watts divided by 12 volts or 37.5 amps. Note that this is too much to draw through a cigarette lighter. For a load over 15 amps I would recommend connecting the inverter directly to your house battery. Most inverters come with battery clamps.
Forty amps may seem like a lot of power, but your alternator should not have trouble keeping up with it. I would not expect you to notice too much additional load on the engine. Be sure and run your engine for a few minutes after you turn off the lights to recharge the battery. I would also recommend that you don’t disconnect the inverter from the battery right away, but turn it off with its power switch if it has one. A discharging battery can give off explosive gas and the spark from disconnecting the inverter could set it off. Open the battery compartment and let it ventilate for a few minutes before disconnecting the inverter.
Hope this helps.