Floor finishes are durable and waterproof, but water will still seep through them if you let it sit long enough. The result is an ugly greyish stain that can spoil the whole floor. You don't have to haul out the big sanders to deal with a water stain. You may be able to remove it very easily. You may have to sand away some finish to fix a stubborn stain, but only enough to expose the stain. When you spread new finish, though, you'll have to spread it over a larger area so it will blend.
Spread toothpaste on the stain and rub it in a circular motion with a damp rag. Afterwards, clean off the toothpaste with another rag. If the stain hasn't penetrated too deeply, this may remove it.
Sand the stain with a palm sander to remove the finish layer. Use 120-grit sandpaper, and if it gums up without removing the finish, switch to 100-grit paper. Sand an area slightly wider than the stain, and ease off with the sander around the edges so they feather into the rest of the floor rather than forming a discrete border.
Mix a 50/50 solution of household bleach and water in a bucket. Soak a rag in the solution, then lay it on top of the stain and leave it for 30 minutes so the stain can soak in. Leave it longer for darker stains.
Remove the cloth and let the wood dry completely. Sand it lightly by hand with 120-grit paper to knock down the grain and remove any residue left by the bleach.
Wipe stain on the repair with a rag and let the stain dry overnight.
Dab a rag in floor finish and rub the finish into the wood. Spread the finish past the borders where you sanded and ease off on the rag around the edges of the finish, as you did when you were sanding the stain. This feathering will blend the repair into the rest of the floor.
Let the finish dry overnight, then lightly sand it and re-coat. Repeat if necessary.
Use lacquer thinner to test the floor finish so you can re-coat it with the same material. Dampen a rag with thinner, moisten the finish near the stain, and feel the rag. If it is sticky, re-coat the floor with varnish. Otherwise, use polyurethane.
If the stain is difficult to remove, don't try to sand it out or you'll create a noticeable depression. If the bleach doesn't remove all of it, you may have to settle for the discolouration that remains.