If you're hoping to take any photos of Niagara Falls from somewhere other than the usual observation platforms, you may want to consider a waterproof camera... going on the Maid of the Mist or on some of the "up close" walks, you will definitely get soaked!
Depending on the wind, you can get a fair amount of mist even on the normal observation areas, but I doubt that'll hurt your camera, just put some spots on your lens.