Go to Wilson Electronics home page ... One of the few things I can recommend with good conscience.
And a fine recommendation it would be, had he asked about cellular. He asked about WiFi, which is a different technology altogether, and not what Wilson makes.
To answer the OP's question, you have several choices, and I agree with Craig that there is lots of good info in this earlier thread:http://www.busconversions.com/bbs/index.php?topic=12178.0
It boils down, really, to two choices. One is to turn off your built-in WiFi card, which has limited range due to the limitations of the built-in antenna, and use an external adapter, either PC-Card or USB, that has a jack for an external antenna. Then you will have a choice of many external antennas that will get you extra range. A related idea, and what I do when I need to, is to use a USB dongle and a long USB extension cord, available up to 15', which will let you put the dongle in a place where the signal is stronger while still putting the laptop on, uhh, your lap. I have a roof hatch above my chair, and just sticking the dongle above the roof usually does the trick.
The other is to get or make a WiFi "repeater." Plenty of choices on the open market, or roll your own. The advantages to this method are that you can more easily make a "permanent" installation using a very high-gain roof-mount antenna, and it will support multiple computers simultaneously. This solution, however, does require you to pay attention to channel assignments.
Here are some resources for both:http://www.radiolabs.com/products/wireless/wireless-marine-antenna.phphttp://www.greatboatgear.com/index.php/marine-wifi