When it comes to taking good water reflections, there are two important steps. One is to expose on part of the water that doesn't contain the actual reflection, yet is still close to some part of it. If your not sure what 'setting the exposure' means, you can find some information on this at manual modes.
Next, set your camera on a high aperture number (if photographing landscapes), for example f/11, so the whole landscape will be in focus. Then before taking the shot, focus your camera on the actual reflection and press the shutter button. You should then find you end up with a perfect water reflection that seems almost mirror image.
Reflections in the water
Camera: Canon EOS 400D / Rebel XTi Lens: Canon EF-S 17-85mm f/4-5.6 IS USM Exposure: 1/125th of a second Aperture: f/11 Focal Length: 35 mm ISO Speed: 100 Exposure Program: manual
Why this image worked
This photograph was taken at 7am on a cloudy day. The time of the morning, as well as the overcast conditions, made it a perfect situation for finding good water reflections at the local Botanical Gardens. I took the exposure on the water on the left, just under the green land reflection (the blue part of the water). I then focused on the reflection itself in the middle of the frame and took the photograph. Due to an aperture of f/11, the whole landscape is seen in focus.