removing water stain from seat

David is right you have to clean the entire seat section or you will really struggle to not to end up with ring stains. The material of the seat absorbs muck and dust over time and it’s that that creates the marks around where moisture has been spilled. By cleaning the entire area the colour you see will be the same.
It’s actually common in the trade for valeters to remove the seats and jet wash them, Not how I’d do it but I’ve seen it done often. You can rent an extraction cleaner for little money. Then do the whole interior. You can use a lot of fluid quite safely and suck most of it out leaving only dampness…. I have an old but efficient Karcher Puzzi 100 that does an excellent job. It does seats carpets even curtains with effective ease. autosmart do G101 which is excellent or Bio brisk if the cars or carpets have had dogs or kids near. Even kids sick is neutralised and smells removed with Bio brisk. 👍👍