A couple of Questions :-
How far from the transmitting stations will you be ?
What channels ?
I guess that you have a power boat - does it have a solid bimini over the fly bridge ?
When in Hong Kong I had a OMNI directional antenna (Brit made designed for the caravan market) It looked like the modern soccer ball and about the same size - it was effective when in sight of the TV transmitting towers.
A direction antenna would be OK if you were a 'live-a-board' - otherwise a Disc type might be more suitable.
Here is an Australian outfit that specialises in antennas for Motor Homes and boats :-