Water damage like a water ring from a cup or glass? Explain more please.If it's a ring left on stained furniture, try rubbing mayonnaise in the water stain with your fingers,leaving it a little thick, let it stay overnight (at least 16 to 24 hours). Then rub off with a soft cloth. It should be gone.
Sorry to answer your question with more questions, but do you have any photos, and what is the surface? Are these solid wood or particle board and veneer?
If it is water damage on the side of the cabinet surface, can you paint it or cover it with a veneer?
I also agree with Steve...we need a bit more infor...water rings are one thing ...black streaks and tannin staining are another completely different animal.
It took me a while to find out the finish. It is Pressed wood with a varnish finish. The water stain is from bath towels leaking down the door of our camper bathroom.
I know mayonnaise worked surprisingly well for a water stain on a wood bookcase in our house. Just rub it into the stain. I hate mayonnaise, so I was glad to get rid of it and put it to good use.
Otherwise, you could paint the cupboard. Not sure what other finishing options you have for pressed-wood-type cabinets.
Pressed Wood + water = pile of wet sawdust.
I have found that some light sanding and re-staining can bring this back to some degree. What is important is that the stain is then "varnished" to protect the wood. I used quotes around varnish for a reason...old school varnishes were great but more modern poly...is a bit easier to get good results with. I would top coat the freshly stained wood ( once you have the colors matched and allow the stain to dry for 24 hours) with 4 to 5 coats of minwax wiping poly. (satin) finish. This stuff goes on very thin so 4 or 5 coats is not a problem.