I mostly agree with the other posts, Leland's advice about getting the exposure right is dead on, and there has already been a lot of good info about lens size. One lesson I learned the tough way concerns the quality of the lens. I use the same Canon body you're using and I think the Canon name brand lenses are the way to go. It's a good idea to shop for the best price once you choose the lens you want, but I think the saying "you get what you pay for" is true with lens optics. And if you haven't noticed already there are a lot of irreputable online vendors out there. Some sell lenses for other markets, i.e. Asia, and won't have a warranty which I think is important considering your going to take it on the river. Good luck!