The easiest way I have found is a HIGH QUALITY fm transmitter.
Splitting off wiring can be higher quality, but it also can be badly distorted, especially if the boats have different head units, and therefore different gain staging... One guy's perfect unity gain setup can be the next guys "hideously clipped on the front end" distortion generator. Success will depend on having similar gain structures for all stereos. Tim White's WS420 makes it real easy to do right if you insist on going the wired route.
On my boat, I use a Zune and the appropriate Soundgate CORE line level adptor/charger. Out of the adaptor I hit a y-cord. One half goes to the aux in on my stereo. the other half of the y-cord goes to my Mobile Black Box Eclipse-4000 stereo transmitter, (
www.moileblackbox.com)...
When we raft up, the rest of the boats just tune to my "radio station" and then each sets their own level. I even created a fake "107.3 The Duck" station ID that plays occasionally, and we also made some fake commercials. It can be fun.
On the list of things to be aware of, you cannot broadcast fake call letters, (KMDF for example) you cannot step on another broadcaster's licensed frequency, and you cannot broadcast profanity, obscene or foul language, and any other material deemed inappropriate by the FCC. Use at this power, and for this purpose is largely legal, assuming you follow a few guidelines, and "broadcast" in the positive interest of the general public. Just do not assume total freedom; I have to believe you can get in trouble... I transmit full well knowing I may get asked about it someday...
FM has its quirks, and done wrong it can sound bad; we have all heard bad wireless FM adaptors. The trick is spending some coin for a good one.
No doubt there are other good ways to do it too; I just share my idea if it sounds like trying...