>>2975135I've never needed an actual ND filter in my life (GND's yes but never an ND), if you want to get long exposures I would recommend shooting when it is darker out, around sunset or twilight. If you're going for waterfalls shoot them when they don't have direct light on them as they look much better this way anyways. You'll likely find that at f8 or f11 you'll probably have exposures of around 2 seconds if it doesn't have sunlight on it. If you can afford a cheap polarizer I would highly recommend it, it makes waterfalls look much better and also takes away two stops of light.
If you want to simulate the effects of a GND you can wave a dark object over the top of the lens during a portion of the exposure. This is a really handy tactic and quite easy if your exposure is a second or longer. Again, shoot during sunset or blue hour for the longer exposures.
As far of the rule of thirds it's more like the rule of turds. Just kidding, it really is a great starting point to creating more dynamic compositions but it's not the end all of it. If you solely follow that rule when composing it will become very obvious as someone looks through your images. I'm a huge fan of bringing the viewers eye into the center of the photo, usually having a subject at or near the center. Oddly enough, this gets achieved with lines and other objects off to the sides or thirds of the frame.
Take this image for example. I find that your eye is immediately drawn to the center, but not just because the sun is right there. The grouping of trees roughly one-third off to the left helps and the angle of the snow-covered slope brings balance to the other side of the image. It's a very center-weighted image with a balance due to the rule of thirds.
I'll post another example next.
