Airport Extreme booster antenna - reach over hills?


I just bought a Airport Extreme base station and hooked it up to my 17" PowerBook, I also have an airport express (on extend option) that has my printer hooked up to it.

What I would like to really do is work from my boat dock which is about 150 ft away and 30 downhill from my office where the Extreme base station is mounted (on a window ledge facing the lake).

Will a directional booster antenna only give me a "straight" signal or is there any way to reach the dock below?

Thanks for you help.