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Since you are going to need power at the top of your mast anyway, why
not run ethernet cable up there and use Power Over Ethernet (POE)
technology to provide power and signal path in one cable? I saw a unit
advertised the other day which was set up to do exactly that:

http://www.sputnik.com/products/aps/ap250.html

If you do a google search for things like... POE wifi bridge etc., you
will find other options and products with similar capabilities.

Based on my own experience, power is important. I'm using a long range
USB adapter (Netgear RangeMax), about 20 feet above water. It can
frequently hear remote access points over a mile away but can not
connect with them due to low output power. I routinely connect to
outdoor APs over 1/2 mile away however if they have a decent antenna.