Another quick suggestion or two on the long exposure/water shots...
Along with using a polarizer and/or neutral density filter, first of all make sure you're using a smaller aperture as well (I didn't notice if anyone mentioned that or not). If you're shooting "wide open", even with a neutral density filter, you can still over-expose your shots. In fact, I've gotten a couple of silk shots of water falls on overcast days without using an ND filter just by bumping the aperture up.
This shot of Wallace Lake Falls (also in Berea) was taken with my old Olympus C-4000 p&s. I was able to get a 4 second exposure on this just by using a polarizer and bumping the aperture up to it's minimum setting (f/22 I think...it's been a few years and I don't even have that camera any more). I probably would not have been able to get this shot on a bright sunny day, but cloudy and overcast were just enough to get a longer exposure. I'm very proud of this shot if for no other reason than that this was the shot that everyone said I couldn't do without a ND filter...and I did. I just love proving people wrong
Now here's another nifty little trick...in lieu of having an ND filter, try shooting around or just after sunset! Very simply, as the sun goes down, you'll need to use longer and longer shutter speeds to get the proper exposure...and it tends to make for some rather surreal colors and such to boot.
Here's one I took that was actually well after sunset...
This was shot at Berea Falls up here in Ohio and was taken well after dark. The only ambient light came from a couple of street lights up at the top of the hill over-looking the falls (hence the orange hue). This was shot at f/7.1, ISO 800 with a 10 second shutter speed...yes those people sat that still for that long! LOL!!! Another advantage of shooting after dark (in my opinion at least) is that you don't have to worry about the angle of the sun or anything...hard light, soft light, morning light, evening light, etc., because...well...there is no sun! I guess one could say that in this case, a good way to get a nice even lighting is to not use any at all! LOL!!!!!!!
Another little tip here...if your shooting digital (sorry...can't remember), don't be afraid to experiment. Try different shutter speeds and apertures and as you review your photos on the camera, you'll get an idea of what's working and what's not. Do remember though that if you do try this at night, that images on a camera LCD will look "brighter" on that LCD at night then they actually are and will end up looking a little under exposed when you get them home on the computer.
I kind of hate to admit it, but this is usually my aproach...I'll shoot a couple of "test shots" and let the conditions dictate my exposure settings (both at night and during the day time as well). I'm sure somewhere
there is a precise mathematical formula for determining the precise exposure settings for a given light meter reading in this kind of situation, but for me it's just easier to "play with it a bit" until I get what I'm looking for...after all, the water fall isn't going anywhere! LOL!!! Unlike trying to shoot animals where "the perfect shot" may pass at a moments notice, in this kind of situation you can take your time and play with things a little until you have it just right.
Now a quick safety tip here...if you do try this "at night", first and foremost make sure you are familiar with the surrounding area and that you KNOW
how to get back out when you're done shooting! If you're not an experienced night time hiker, it's very easy to get lost even if you do know the area well. I mention this because sometimes even us folks who have been doing this for a while can have accidents...these are the exact same falls in the photo where I fell thru the ice a couple of months ago! I do know this area very well, but sometimes sh*t happens when you get careless. Of course, make sure you bring a flash light to boot...and make sure the freakin batteries are charged! I've had that happen too...once on a night time hike with my wife out at Findley State Park, our flashlight batteries went dead about half way thru the hike and we ended up finding our way back to the car in pitch blackness. I'm not kidding either...moonless night in the woods and we literally could only see 2 or 3 feet in front of us. Fortunately, we had our dogs with us, so we just told them to find the car and we followed them...if it hadn't of been for the dogs, we'd probably have ended up spending the night in the woods. Again this was at an area that my wife and I have been to many
times. Things change after dark and it's really easy to get disoriented very quickly.
With that said, shooting water/falls after dark (or just before dawn if your a morning person) is a great way to get those real "silky" shots without having to invest in an ND filter.