Two products spring to mind:
It is possible to buy filters that you put on the end of your hosepipe which filter out the stuff in the water that causes the water marks. You should be able to get one from a good detailing supplies website.
Also, have you thought about using a Quick detailer after you dry the car? I am currently using Meguiars Final Touch. It's about £16 for a gallon but last for absolutely ages. You'll need a sprayer to put in it (£3). Just spray onto the paintwork and wipe off with a microfibe cloth. Gets rid of watermarks and leaves a lovely wet look shine on your paint. Plus it only takes minutes.
This product is saving me loads of time. It means you can get that fantastic finish that you get from a 'big clean' but in much less time so I only have to 'top up' my wax layer about once a month.