Forget directional antennas on a boat
unless you are on a dock. Boats are usually dynamic state, They move all over the place when not at a dock. Directional antennas have to be aimed at each other. Think of a spotlight vs. a flood light. Whgat works best for a boat is a dipole antenna. Sort of looks like your VHF antenna placed up high in the rigging.
That said contrary to what has been said before most WIFI cards do not come with a jack for an external antenna. The exception is if you have a desktop computer onboard.
I found the Buffalo card with a 9 bd gain antenna will give me an effective working rangs of 1.5 miles max. The cost is about 150.00