So, there you are, cruising along at 36,000 feet and the cabin door blows out. Will your watch crystal pop out or not? Would you really care? If so, read on . . .
The force on the crystal is the pressure differential between the outside and inside of the watch, multiplied by the area of the crystal.
Most planes are pressurized inside to about the equivalent of 8,000 feet and airliners fly at around 36,000 feet, so therein lies the pressure difference which is about 52700 Pascals. See here for an ICAO standard atmosphere calculator.
My Gallet Navigator's watch, for example, has a crystal of 30 mm diameter.
I figure the force to be 37 Newtons or 3.8 kg or 8.4 lb. Not a lot, really. Therefore it's probable that the crystal will stay in place, although your ears won't be up to much for a while!