What's the optimal placement for water pumps?

As I remember it, water pump placement does affect certain aspects including but possibly not limited to:

• If placed within the pollution area of industrial zones, the water is polluted.
• If placed near landfills water is polluted.
• If placed near 'Dirty' (Coal for example) Power Plants, water is polluted.
• If placed near residential zones, property value is lowered.
• If placed near water, output is increased. (I remember this being the case more clearly in Simcity 2000, however (Thanks to @Private Pansy) this doesn't seem to work in SC4)

Generally following these guidelines they are best placed somewhere near a natural water source some distance from the main body of your city. This is less of an issue than water towers because the distance the pumps effect is mitigated by piping. (Just remember to provide them with power.)

As stated in my comment, take this answer with a grain of salt. I've written it entirely based off my memory and cannot prove or source any of the information.

You can place pump anywhere where there's no water pollution. That's pretty much the only thing that matters with regards to placement of water pumps and towers.

Avoid placing pumps and water tower near areas of heavy water pollution, such as farms, heavy industry, polluting power stations, and landfills. Use the water pollution Data View to find areas without water pollution. Pumps placed in polluted areas will suffer from lower output and may shut down entirely. However, do remember that pumps require power to operate.

As an aside, only commercial and residential zones suffer from a lack of water supply. Connecting industrial zones and power plants with water pipes only reduce their likelihood of catching fire, and is usually not considered 'worth' the additional water usage to connect.