Getting a watermark out of a cherished oak table doesn't necessarily mean having to strip and refinish the entire table. Before you have to resort to stripping and refinishing the entire table, there are ways to do this with items you most likely have around the house. The key is to replace the oil in the wood finish that was soaked up by the water.
Spread mayonnaise or olive oil onto a paper towel. Place on water stain for 15 minutes. Remove paper towel. Repeat until stain is gone.
Place smooth cloth over the stain. Heat with hot iron set to dry (no steam) on its lowest setting for 3-5 seconds. Lift cloth. Repeat until stain is gone.
Dampen a Mr. Clean Magic Eraser and gently smooth over the stain in the direction of the wood grain. Repeat until stain is gone.
Dip a clean, lint-free cloth into some oak wood stain. Dab onto watermark and gently smooth in the direction of the wood grain. Let dry.
Completely strip and refinish the table. Rub sandpaper over the wood in the direction of the grain until all the finish is gone and you are left with bare wood. Wipe the surface down with a clean, lint-free cloth to remove all traces of sand and dust. Choose a polyurethane stain in a tint you like. Paint the stain evenly and smoothly with the grain of the wood and allow to dry completely. Paint several thin coats for a darker stain, allowing it to dry between coats.
Treat watermarks as quickly as possible.
If the table is an antique, consult with an antique furniture expert before treating the stain. When using wood stain, always work in a well-ventilated area. Stripping the table should be a last resort for water stains, and if you have never stripped and refinished furniture before, have a professional do it for you.