First I think the cable supplier might get really upset that you are "broadcasting" a signal, after all that's why there is "cable".
2nd, I believe what you are attempting to do is illegal, I don't think you want the FCC after you.
I looked at the X10 and this appears to work from the output of a receiver (cable box, satellite box, etc.) I don't believe it will work directly on a cable connection but if you have a cable box with a remote then this would be the best way to go.
BTW it must be some boat to want to move the TV to different areas (:. I had a 28 footer (read sold) and I had only two areas I wanted a TV connection, on deck and in the rear cabin. For as many times as I used it I could have just temporarily ran a coax on the deck......
Joseph Golan, RCDD