Water (vapor) under the crystal
Water condensation on the inside of your watch crystal means that moisture has entered your watch, most likely through the hole in which the setting crown/stem is fitted, or, possibly, through the door of your battery compartment. It's usually easier for water to enter a watch then to leave it again. Water inside a watch can quickly corrode delicate parts, causing erratic timekeeping and premature failure of the watch, so you'll want to get it out of your watch as quickly as possible.
Fortunately, it's not that difficult to get moisture out of a Swatch. Pull the crown out into the time setting position and remove the back battery hatch. Then, turn the watch face down and leave it in the sun for a few hours. If you're worried about dust and/or rain getting into your watch, leave it inside a parked car that's in the sun--that will actually speed up the drying process, as car interiors, with the windows closed, quickly heat up far beyond outside ambient temperatures (except in winter). If it's dark outside, you can accelerate the drying process by using a hair dryer set to low / medium heat and heating the crystal gently until the moisture has evaporated. You don't want to heat the crystal too much or else it may start to melt or crack.
Once the water is out of the watch, close up the battery compartment and update the time/date for your watch. You should be good to go, as long as you try to keep your watch dry when doing things like washing hands and the like. If you had been doing so, it's also time to stop wearing your watch when you shower, wash your car, or engage in other activities where the watch is exposed to a lot of water.