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Technically, you're looking for the highest 'gain' possible. Though, you may be able to take that with a grain of salt. Here is a good link to describe gain (and more than likely muddy the waters):
What is TV antenna signal gain?
Omni-Directional is going to be important, particularly if you intend to swing on a mooring or anchor while watching tv. Technically height is also going to be an issue as the broadcast signal is line-of-sight and a boat is typically as the lowest possible point on the horizon being at water level. An antennae mounted on a sailboat mast, or a post from a flylbridge will perform better than on the arch of an express. However that is certainly not to say that it won't work. Only that they may get better more consistent resolution and more channels than one mounted lower.
Consider an Apple TV. This can be coupled with an unlimited data plan on you phone. Turn your phone or device into a hotspot. Connect the AppleTV to your hotspot. now your phone or device is acting like a router. If you have DirectTV, Comcast or some other TV provider at home, then use your logon. For example, I can pull up any number of channels (e.g. Hulu, Netflix, HBO, History Channel, CBS News). Enter either your TV provider logon or your acct logon (hulu or netflix) and away you go. We were watching Game of Thrones on the anchor just the other weekend.