What's the optimal placement for water pumps?

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.