You can get one at Radio shack, I have one but have never tried it on a boat. It's about 3"x5"x1" and has one output, three inputs with buttonw on the front. Should work fine, and better than digging under the dash swappin RCA's! You will have level issues due to the difference in output from the head unit to the MP3 player.
If your head doesn't have an aux sometimes you can get a pigtail that plugs into your CD changer plug that will give you an RCA input. There is one made for most brands of HU.