Getting Free HDTV on a boat is actually just the same as getting it on land. You just need an antenna. Pretty much any outdoor antenna will do. I personally don’t recommend indoor antennas because they aren’t waterproof and because the lacquered wood used on many boats does a great job of rejecting signal.
You should get great signal while at sea as long as you’re within about 50 miles of the broadcast tower. With nothing to block it, the signal should travel easily. The only issue is that HDTV over-the-air signals don’t work at all if the vessel is moving faster than about 10mph. This is a limitation of digital broadcasting whether on land or sea; it’s not specific to boats. So, while you’re not going to get great reception while moving, you’ll have no problem while anchored for the night.
Over-the-air TV is great if you’re taking a long cruise up or down the coast – just remember to scan for channels whenever you set down for the night because obviously, the channel selection will be different no matter where you go.