Our friends in Mexico have had good success using a wifi antenna "Sigma 6" purchased at Custom Marine Electronics in San Diego, around $200USD, I believe.
I get the impression, though, that you're trying to set up something more sophisticated.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)