you can only get full effects with a DX9 hardware compatible card, since some of the effects PW uses are only available in DX9. If you have an older card then you won't be able to see the difference.
btw water effects in PW are not really optimized: i get drop of 10 fps when i set them to high on a 8600mGT. the worst is, i get the fps drop even when there is no water around -.-