Look at the Boston Acoustics Voyager. I have a pair of Voyager II's (I think thats the name) at my cabin, and I have been really impressed with them. I've had them for around 4-5 years now, in a pretty harsh environment, and they don't show any signs at all of wear.
One tip when looking at outdoor speakers, I wouldn't even bother considering low bass response. Low frequencies seem just about impossible to reproduce outdoors in an unenclosed environment, focus on the speakers ability to cover the rest of the range.