Water stains on car seats, either cloth or leather, are not a big deal; you can easily clean them at home without professional care. You can take care of fairly small stains, but if you have extensive water damage to the interior of your car, you may need to hire a professional.
You will need to have a few things ready before you get started on the cleaning process. Some of the items are probably already sitting around in a cupboard, and some you may have to buy. You won't need to spend a lot of money to clean your car seats at home.
Give the seat a thorough vacuuming, because even tiny bits of dirt will start to show up when you apply the cleaning product -- you don't want to make the stain worse. If you have a hard water or saltwater stain, rub a teaspoon-sized amount of vinegar into the stain with a dull rag or microfiber cloth and let it sit for several minutes before applying the shampoo. The vinegar will help to dissolve any salt or minerals that may otherwise be left behind after you remove the stain.
Apply it to the whole seat. If you try to clean only the stain, it may become even more noticeable.
Gently brush the shampoo into the seat, taking care not to damage it.
Remove the shampoo with a towel and allow the car to air out.
Leather is very resilient, if it's real leather, and you may find that it simply dries on its own after a while and does not leave a stain. If it has been some time, though, and you know the stain isn't going away by itself, take action. Vinegar is safe to use on leather for cleaning hard water stains.
Get a clean bowl of room-temperature water and a clean cloth.
Soak the cloth, then wring out most of the water. You want it damp, not wet.
Start rubbing outward from the stain to the seams of the seat on both sides, covering the entire area, so that you don't make the stain more noticeable.
Wipe the cleaned area with a dry cloth, allow it time to dry naturally, and then use leather conditioner to finish the job.
After following this guide, if you still can't get rid of the stain, you may need to have it professionally cleaned. If you clean cloth seats and the fabric feels a bit tight or hard afterward, give it another go with the vacuum and it should be as good as new.