An automatic dive watch is built to be water resistant. Because they are taken out on dives, they can withstand pressures of diving up to 660 feet. But, these watches are usually worn outside of the water as well where wear and tear can occur. Yes, they are water resistant but even an extreme change in temperature or pressure can allow water to seep in.
The first thing you need to do before even touching your watch is decide whether it is worth you fixing it yourself. If your watch is very expensive, you may want to go to a professional to have it fixed. Places such as Fossil are willing to fix your watch for free due to their lifetime warranty. However, depending on the brand of automatic dive watch you buy, they may have a limited warranty which can be made void by opening the back of the watch face yourself.
If you only have a bit of moisture in your watch face, there are things you can do without even having to go into the mechanics.
One of the first things you can try is leaving your automatic dive watch out in a dry place and preferably somewhere with sunlight. Place the watch over an absorbent towel to aid the removal of moisture. The heat of the sun will evaporate the moisture.
You can also try taking a hair dryer (placed on the lowest heat setting) or a fan to remove the water. Many hair dryers now come with a setting that allows for air to blow out at room temperature and this will be the safest way to do this. The last thing you want is to melt any of the sensitive components because of high heat. This can also work with air vents or radiators.
If that does not work, you take a big bag of dry rice and place your automatic dive watch within it. Rice loves moisture and by leaving your watch in it for two to three days, the water may disappear.
Lastly, you can try to do it yourself. I suggest purchasing watch repair tools. You can find these at your local jewelry store. When you have the proper tools, take the back of the watch off as if you were to change the battery. An automatic dive watch does not have a regular battery like most watches so do not be shocked. Let the watch dry in open air and when you see absolutely no moisture, replace the back and the seam carefully as to not scratch any of the mechanisms.
These are just some ways you can try to remove the water out of your automatic dive watch and is in no way a guarantee. If you try one (or all) of these methods and there still is water in the watch face, take it to a professional. This is especially true for salt water because salt water and rust and damage the components.