Speakeasy has been sailing around Lake Michigan, but is now on a can in Monroe Harbor, which has pretty good WiFi access. With a boost from The Wirie, we get 5 bars even below deck. The basic ingredient of The Wirie is an Alpha signal booster, which can be taken onshore and used at Internet cafes and other locations.
The New York Times reports on another device that might work on your boat without needing to install an external antenna. I'd like to hear if anyone uses this in their boat.