Did a guest forget to use a coaster and leave a water ring? Watermarks are certainly unsightly but can be removed if action is taken as soon as possible. The longer the water is allowed to settle the worse the damage will be for the wood.
Remove Recent Watermarks
Hair Dryer Method
If the damage was done within the last 24 hours, then the hair dryer method can be effective. The basic premise is to heat up the surface of the wood and get the moisture to evaporate. After fixing the spot apply a polish or wax to the wood to protect against further spills.
Set the hair dryer to a heat setting and slowly move the air stream around the water mark. It will take some time to heat up the surface. Be sure to exercise caution so additional damage is not done. Heat will expand the wood and it will have to contract again as it cools. This expansion and contraction process could perceivably create a crack.
Lay a towel over the damaged area. Move an iron over the cloth slowly for several minutes. Pick up the cloth and check the spot. Repeat the process until the water has evaporated into the towel. Apply a polish or wax to protect the wood from future exposure to water.
Removing Tough Watermarks
This method of soft abrasion comes from eHow. Gather tooth paste (plain version with no added whitening or gels), baking soda, and a cloth. Mix equal parts of tooth paste and baking soda together. Dampen the cloth into the mixture and gently rub it into the water mark. Repeat this process until the mark is removed. Again, use a polish or wax to protect the wood from future harm.