Composition is always going to be a matter of taste and often experimentation too, trying different focal lengths to see what works for you, so I won't comment on that, particularly as it was something you were trying for the first time. It's always good to step outside your comfort zone every now and again. I think you've got a good balance with the shutterspeed, although again, it's worth experimenting, as not everyone likes a misty or cotton wool effect and for some shots you need some texture. For me though, it is at least a stop underexposed, if not two. When you have water movement, it is important that the water is near white (unless there is colouring in the water, for example in peaty areas), although you have to be careful not to blow the highlights in the fastest moving areas. As a learning experience, try increasing the exposure, so that you can get a better idea the next time youj try it. You may need to play around with apertures and even ISO to get it near enough right in camera.