I'd try a little oil, just on a test spot to see if that works. If it has penetrated you may have to dig deep to catch up with it. Lots of sanding and the right oil may float it out. Look up orange oil online. There are many positives and a few dissenters. I have a colleague who swears by it. - PJA
A light sand and varnish? If it’s an old floor, though, it will be very difficult to match the colour. - MartyH
White spirits may do the job. Turps is stronger and both will do some damage to the sealer that may be covered up with a wax polish. You want to have some small areas to test on first.
Thinking out of the box, if a mark has just etched the sealer rather than staining the wood you could try using a car cut and polish to get down to the good sealer without removing it all. Again, test on a small area first and this idea will only work on a gloss finish. - Brad
I was going to suggest acetone (nail polish remover) but then thought it may be too much. Maybe try tea tree oil? Nothing lost, nothing gained, so they say if you think the next step is to sand it back. I don't envy having to do that. - darylhewston
You can try the old-fashioned way of removing watermarks from good timber furniture. You need brown paper and a warm iron. Put your paper on a small area of the mark, the size of your iron and then place your very warm (but not hot) iron on the paper and hold it there for a few minutes at a time. Keep checking how you are going. You should see the mark shrinking. This has worked for me on my very good dining table, etc. - Sillabil