Most new head units have an AUX input. Some are RCA and some are 1/8" headphone jack input. Depending on the amount of boats I can just run them all off my computer, or I have a line driver that I use to boost the power from the computer. I have done 8 boats off just the computer with no problem.

I carry an array of RCA cables, RCA splitters, RCA Y cables and 1/8" adaptors. We just go from boat to boat with an RCA cable and connect depending on what input their radio has.