If you want to get serious about your photography you will want to get used to changing lenses. You get better at it over time and it becomes second nature… Take a look at how professionals do it. Just look it up on youtube and there are videos showing how to change fast.
For the water shot you might also need a neutral density filter to make it so less light gets in… Honestly the best way to learn this is to learn from other people. Just google something like “how to photorgraph a waterfall” or something like that.
I like this lens:
http://www.amazon.com/Canon-70-200mm-2-8L-Telephoto-Cameras/dp/B00006I53X
It’s a bit pricey though 😮