I just looked through a lot of different catalogs and it looks like you will have to buy a unit with either a CD or cassette, the cassette option is usually less expensive. In Overtons.com there are several units with the MP3 jack capability, even one unit for only $59.99 with speakers (I can't vouch for their water resistance). Try Overtons.com, WestMarine.com, BoatersWorld.com, BassPro.com, etc. They all have many different options. I have a very nice Jensen marine CD receiver without the MP3 jack that has good anticorrosion construction but only cost about $120 so expect to pay more for the MP3 jack. Good luck!
