You should do the exact opposite thing. Washing before claying is much much much MUCH MORE IMPORTANT than after it. If someone does wash the car after claying, like I do especially when doing Opti-coat its totally an optional thing.
Clay hardly leaves any residues and these are usually removed once you end up wiping the car down or using a glaze.
As far as the reason goes of ONE watermagnet by the time you will be done with the whole car the watermagnet would have for sure dried. Keep it underneath a fan if you think it wont dry outside.
Get a detailer if possible for the claying process. Will help. Best of luck for the claying process !!