I grew up in Niagara Falls (Canada). Generally speaking, it's frowned upon to shine lasers at and around the falls. People have done it before. I've never seen it myself, but you hear about it once in a while. There are quite literally thousands of people there all the time. You draw far too much attention to yourself, you'll immediately have at least a hundred people surrounding you asking you questions, and you're usually asked to stop by the police. It's not illegal, and I don't think anybody was ever charged with anything, but the authorities don't like it. Perhaps because it's potentially dangerous, and in such a crowded place, I'm inclined to agree with them.
Other things to keep in mind include that there are lots of low-flying helicopters in the area that you need to watch out for. Furthermore, in the rock behind the falls (beneath the upper river), there are tunnels and windows with people in them who could also potentially be at risk to high powered laser if they were shined into the falling water at the right angle. Additionally, there are quite a few trails, paths, roads, platforms, and ledges at all different levels between the upper and lower parts of the river, all of which may have people on them at any time.