I use a handheld Bissell cleaner for spot cleaning on upholstery. Put a carpet cleaner formula in the machine and spray with the nozzle. Lightly scrub and coat the ENTIRE seat with the cleaner and suck up all the liquid. It does wonders to remove stains and you can also use this method on rugs too. Good luck!
A white or light towel with water that you brush over the stains, dampening them to spread out the ring left by the water. They also make upholstery cleaners you spray on you can get at Walmart or any car parts store. Directions are on the bottle.
You could try this: https://www.hunker.com/13421128/how-to-remove-water-stains-from-car-upholstery