We use a small rectangular antenna from Wal-Mart. Most of the time it just lays down on top of the TV, but if we need to, we can place it on top of the cabin, next to the companionway. This only works in a municipal area where there are lots of stations available (we are in the Boston Harbor area). I'm pretty sure the antennas are direction sensitive. When we lived in Englewood, FL, our dock mates would mount the same antenna we have now onto a pole and secure that to a piling. They would then orient the antenna toward Sarasota or Ft. Myers, since they could only pick up one or the other, but not both at the same time. They found that if the antenna was mounted on the boat and the boat rocked, they would lose the station.
One neighbor hoisted his up the mast, but it banged around a lot, and didn't maintain reception. He than mounted it on a pole on a piling. I think if you want to go with the mast mount, you have to invest in one of those expensive dome type antennas that automatically locks onto a station.
We only use our TV while tied up to the dock and on shore power. I have to say the HD quality of the pictures is great. Every bit as good as the DVD. I have to laugh when I see the TV commercials that portray these antennas as something new and magical. "No more cable!" they say. Well, yeah, it's a TV, so ....