What shampoo are you using?

Its the calcium in the water which causes that, you should use something better to wash your car with like this http://www.cleanyourcar.co.uk/washin.../prod_144.html
Its meant to neutralize the calcium in the water to stop water marks and it does, plus its way better than anything else ive used, including autoglym.

Plus you should always dry any water off, leaving it is not good long term, but using something like the above should almost stop any marks at all, but you have to help it along by drying it well aswell