It depends what you want to do ...
This is a confusing topic, and there are several ways to go about it, so we'll need to ask a few questions. Then the board's experts can wade in with their advice.
Do you want to:
1) connect one or more onboard PCs (wireless laptop) to a marina wireless system for Internet access & e-mail?
2) connect two or more PCs onboard to each other? to onboard navigation systems?
3) carry your laptop to a nearby Starbucks with a Wi-Fi hotspot to access the Internet & e-mail?
4) do something else? (Access your work e-mail? - We can't help you with this unless you're a live-aboard. Violates the relaxation clause of the Imaginary Ericson Sailboat Operators Agreement)
5) all of the above?
Have you looked into the cost & coverage of Internet access from a cellular phone provider?
I am not a Wi-Fi expert, but the wife & I survived two weeks in Washington DC, Alexandria VA, Kill Devil Hills NC, and Annapolis MD with a Wi-Fi capable PDA. Never missed an e-mail and kept up with Calvin & Hobbes. (We were not on a boat though.)
I hope we can help you figure this out. Plenty of people are wondering the same thing.