Using an USB adaptor with an antenna may not be enough for that range. I can't really make an accurate prediction about which USB antenna to use because certain ones work better with different routers and the drivers come into play also.
It may help to get one from the same company as the router being used although there are too many factors involved to predict the outcome. Wireless G has it's limitations which is why there is wireless N.
If you want it to work right and don't mind spending some money to upgrade the hardware then going to a wireless-wired N router would be the way to go. You would need to run an eithernet cable from the router to a wireless N access point at the closest location to the pool house for the best possible connection.
This would give you the best chance at making this work but I realize it's probably more work, time and money than you want to put into it. You would also need to use wireless N capible network adaptors on the pool computers to pick up the signal. This type of set up would work for certain but it's still a matter of how much you want to put into it.