All the boats have RCA line inputs or some have the 1/8" stereo jack for connecting something like an MP3 player. I just carry several 20' RCA cables and then have Y cables that we can go from boat to boat. Also have the adaptors to go into the 1/8" jacks.

I have VirtualDJ loaded on my laptop. I just setup a playlist and we go all day without having to worry. That goes out to a 5v line driver and then off to the other boats. That way everyone can control volume at their own boat.

Check out www.virtualdj.com there is a free home edition

You mentioned the boats have inputs. Are you using the RCA Output?

Agree with what Chargeit says, no matter how you do it, there's gonna be a loss of loudness or quality. It's better to be cabled but not everybody has that option.