While others have mentioned the Engenius 362 I prefer a different model.
The Engenius EOC- 3220 ext is about $120 and is weatherproof. This is directional but comes with a little antenna that makes it omnidirectional. This thing will pull in wifi signals that are miles away. I guess it's being replaced by a new unit ( EOC-2610) that's more powerful and even cheaper. But I don't know much about it.
While you could mount this up the mast you only need a few miles line of sight. I attach mine to the stern rail and turn it by hand to use the directional antenna while sitting with my wireless laptop (I've noticed that a standard beer bottle assists this process). I sometimes raise it on a long boat hook in harbor but I honestly don't see much different. I've also heard of people just using a flag halyard to lift it aloft.
By putting the smarts at the antenna this avoids the severe loss of trying to move wifi signals through coax or the length limits of usb cables. You connect using a standard cat 5 network cable that can run 300ft+.
While the Engenius can supposedly be it's own wireless hub the setup is geeky. Instead get a cheap $50 wireless router (linksys, netgear) like you would use on a home DSL setup. Plug the Engenius into this.
You should also be aware that the "golden days" of pulling into a harbor and finding free wifi is going away fast. Encyrption is the norm. You'll be paying someone more often than not.
I didn't know about these guys when I bought mine but I wish I had. They clearly understand boats. No relation.
Engenius wifi setup for boats eoc-322 ext eub-362 ext
Carl