If you're willing to noodle around with router software, you can do this with cheap, off-the-shelf wireless router.
My boat is about 400M from a commercial wi-fi access point on the bluff above our marina. My laptop couldn't pull in the signal reliably so I bought a Cisco/Linksys WRT54GL at Fry's for about $70.
Most consumer routers are designed so that the "internet side" of the router is hardwired to the internet and the "home side" of the router is wireless. On my boat, I needed the opposite. This isn't a limitation of the hardware. It's a limitation of the software that Linksys includes with the router.
The great thing about this particular line of routers is that there is an active community of people who have created new software to run on them. One of the most popular is called DD-WRT. The folks who have contributed to DD-WRT have expanded the capability of the router far beyond what Linksys provided. And it's free.
So I downloaded and installed DD-WRT on my Linksys Router and set it up so that the wireless side of the router connects to the wi-fi service on the hill, and the mac just plugs into the normal ethernet hub build into the router.
I'm happy with the results. The small dual antennas on the router are enough to get the job done with the router just sitting down below on the nav station. About a week after I got it working, I improved the system so that the "home side" of the network is also wireless! So my private network on the boat is wireless, and I'm connected to shore-based wi-fi. I'm surfing just fine!
So these are the advantages for me: It's cheap. I learned some new things and now I have an intimate understanding of how it works thanks to the DIY nature of the project. It attaches any number of computers to the wi-fi service. The router's wall wart outputs 12v, so I have the option draw router power off the battery. I can also add a "real" antenna for extra signal strength if needed later. (Haven't needed it yet).
Disadvantages? It took a couple evenings to get it working. You can't call Linksys if it doesn't work and it might require some basic computer networking knowledge or the desire to learn.
I didn't know if this would work when I started, but at $70, even complete failure wouldn't have broken the bank. At worst I would have to forgo a couple bottles of good wine. Thankfully that didn't happen. :-)
Last edited by normdeplume; 04-06-2010 at 12:49 PM.