If it was clear water, chances are what it’s done is left a clean patch, with a white salty ring round the edge.
Shampoo and extraction Vac is the best way to actually remove the stain. It’s staggering the amount of dirt that comes out of even a clean looking seat.
Otherwise, a bowl of warm water with washing powder (not much)
Clean sponge and give the seat a good scrub (all of it, probably both front if you want them to match)
Do it on a hot day and they will dry fairly quick.
(£30/£40 if you want a valeter to do a proper job with an extraction Vac)
Or hire a Rug Doctor machine and use the hand attachment. Do the house carpets while you have it.