the only way you will have 120 volts is to have 120 volt source. just putting the receptacle in won't do it. you could get a 12 volt-120 volt converter and then plug your accessories into it, but thats not energy efficient. use 12 volt bulbs and wire in a terminal strip with each on a separate fuse. that way one short won't shut them all off. go to an electrical store and get a piece of din rail,6 fuse holders that use glass fuses and 6 terminal connectors and jumpers for both. run 2 #12 wires from the battery or existing boat source to these terminals. positive to the fuses and negative to the terminal strip. then land each light or whatever device to your sources, negative and positive.
or bring it by the house and I'll do it.
PM me and I will draw it out and mail you the sketch
jc