I installed a wireless lan at a marina near Midland, Ontario, Canada.
I used a Linksys access point coupled to a 18 Dbi gain flat plane
array antenna through 30 feet of LMR 400 coax. My slip is about 500 to
600 feet away.
On the boat side of things, I built in one of those all in one
motherboards and used a Linksys pci wireless card.
The trick is to use an external gain antenna on the boat. Omni or
directional works in my case. When at the slip, I just put the antenna
outside so that it has a clear line of site to the base antenna at the
marina office. The Linksys pci card on the boat has an external SMA
reverse polarity connector (the connectors for the LMR 400 coax are
sold at RF Connection) I have relaible connectons as long as the DSL
modem does not crap out! I see 50% to80% signal strength at the slip
using this setup. Some people have tried it using only the dinky
little wireless cards for laptops. This will not work unless you are
really close to the acces point. The laptop wireless cards MUST have
an external antenna connector (coupled to a gain antenna) to make the
range. Oronco cards in my experience work the best. (Silver or gold,
it does not matter)

Tim, Grampian 28
Experienced "Keel Sounder!"