I would listen to Jeff Siegel. An excellent article on WiFi from the boat is by Ben Ellison and also refers to Jeff's experiences:
http://powerandmotoryacht.com/columns/e ... ectronics/
Another source of information is:
http://wireless.wikia.com/wiki/Wi-Fi_on_a_Boat
If I was looking for increased reception, I would start with a Yagi antenna and input that into a reciever/transmitter--bidirectional amplifier (basically a repeater) on the frequency you are picking up. The yagi is best on the 2.4 gig, the parabolic may be better on 5 gig signals (The lower speed 802.11 b, g) are in the 2.4 gigahertz range, then. & N is 5 gigahertz range.
The 802.11 N is inherently a stronger signal, and will have a better range. The latest Mac Books will recieve both b g and N. signals.
Google :802.11b and 802.11g bidirectional amplifier
Basically you have an external antenna on the cabin top, a short cable to the bidirectional amplifier and a second small antenna in the boat which transmits to your Mac Book. Since this is the strongest signal, it will be easy to pick up. All of this will be less than the $350.