Last year on holiday in the Caribbean and wearing one of my vintage watches I was caught in an unexpected, massive storm on a small but speeding ferry between islands. There was little shelter on board the ferry with the rain and wind sheeting through the decks where the passengers and crew were trying to shelter behind flimsy screens. It was only about 40 minutes but we were all pretty soaked until the storm passed and we were back in the usual warm sun. I had tried to protect my watch from the rain (and likely salty spray), but we all were just wearing shorts and T-shirts!
Back in my cabin (it was a cruise holiday) the watch showed fogging under the crystal. Not having any tools with me I immediately dried the case with tissues, pulled out the crown and then carefully (not too close) plied the watch with warm, dry air from the cabin's electric hair drier directed mainly at the crown. I was trying to create a very dry atmosphere around the watch to "dilute" and "draw out" the moist air inside the watch case. It worked but took 2 to 3 + hours of persistence. I was trying not to cook the watch because of damaging the oils. The condensation would disappear a short while after the warm air was applied only to reappear when the hair drier was removed and the case cooled. However each time the condensation reappeared it was slightly less than before, so I reapplied the warm air and so on until all signs of the fogging were gone. I then wore the watch with the crow out, whereupon a little fogging came back (body warmth verses cabin air-conditioning, I assumed) and then carried out the hair dryer treatment again. Eventually all the moisture/fogging was gone. The watch continued working during all that time. I continued to wear the watch for several hours to ensure the case kept warm and ensure no further fogging.
When I returned home I immediately opened the case and there were no visual signs of moisture, discolouring or corrosion and I checked the hairspring with my loupe. I placed the opened watch inside a warm airing cupboard for several hours and then inside a closed box with silica sachets for a few days. The watch continues to work and keeps good time as before. I was lucky but what else could I have done!? Perhaps take a case opener on holiday, but where does that stop?
Best
Clive