XM radio and Boats

I have a couple of questions about putting an XM unit in my boat. I have the SkyFi2 in my car and I was wondering if you really NEED to buy the marine kit ($200). I was thinking I could just buy another car kit ($60) and mount the antenna on top of the tower and wire it through the tower with the speakers I got up there. Also, the head unit I just picked up has the RCA inputs, so can I just plug the cradle right into the deck with an RCA-to-headphone jack cord? The fm modulator is ok, but the sound quality isn't the greatest. Anybody with any info to help me out would be great! Thanks in advance, I know there are quite a few stereo geniuses out there who know way more than I do!