My new WiFi adapter for the boat arrived yesterday and I am very
impressed with the early test results. It is a high power USB adapter
with provision for an external antenna, and not particularly
expensive:

http://wlanparts.com/product/NUB-362EXT

I'm using it with an 8.5 db omnidirectional antenna mounted about 25
feet above the water and fed with 30 feet of low loss coax.

The drivers installed very quickly and cleanly, and the client
software allows scanning and selection for all available access
points.

It is very significantly better than my previous best, which was a USB
Rangemax adapter mounted about 20 feet above the water.

Thanks to Glenn Ashmore who pointed out this unit in a previous post.