How do I remove a water stain on wood flooring?

You'll have to see if you can refinish or may have to replace those boards.

http://www.howtocleanstuff.net/how-to-remove-a-dark-stain-from-hardwood-flooring/

Google wood bleach then Google Oxalic Acid.

There are commercial products like a one made by Savogran.

Wood bleach is not bleach like Clorox. It is made with Oxalic Acid (not a strong acid).

I would and did by Oxalic Acid from EBay but you also get it on Amazon. It's a lot cheaper than the commercial stuff. Plus you can use for other things. I.e. I used it to clean rust stains on my concrete driveway.

It comes in powder form and you mix it with water (preferably distilled) then apply. You'll need to neutralize the Oxalic Acid before finishing the wood. Easy enough with baking soda. You can Google complete instructions on all this.

Here's one link of many when I Googled the subject ...

https://www.highlandwoodworking.com/woodworking-tips-1111nov/alannoel.html

Wood bleach them https://www.woodmagazine.com/materials-guide/finishes/get-the-color-out-with-wood-bleach

They sell bleach specifically for wood floors. It is not the bleach you get for clothes. You can find it in the big stores, especially if they sell flooring.

I used peroxide and it worked great.

I have the same problem. I have not fixed my floor, but I can tell you sanding does not help much. I used 40 grit sandpaper. It helped some but not enough to really work. I am going to replace the boards. Good luck..