If your watch crystal fogs up between the watch face and the interior surface of the watch, you have two problems. The first is the moisture that is present inside the watch. This is a simple problem to remedy. The more pressing question is, “How did the water get into your watch in the first place?” After you clear the water from your watch, you should take it to a watch repair specialist to prevent the problem from happening again.
Place the watch, crystal side up, on a warm window sill. As the warmth penetrates the watch, it will vaporize the water, increase the vapor pressure inside the watch case, and force the moisture out.
Alternatively, remove the back of the watch case and place the watch backside up on the same window sill. With the back removed, the water will easily evaporate from the watch and the internal surfaces.
Take the watch to a repair shop or jeweler. Especially with expensive watches or watches that are advertised as water-resistant, have a conversation with the jeweler about the conditions that caused the moisture buildup in your watch, and determine if any repairs are required on the timepiece.