Sorry for reviving such an old thread, but I thought I would give some input to this suggested MacGyver testing method. I would say that the method (theory) does not work, and don't try it!
Why?...
Well simply because heat transfer is not instant "enough". (moment of inertia, kind of...) It would take a few seconds until the heat reaches the inside of the watch enough and to heat the air to expand. Within these few seconds water will have the chance to slip inside. In other words, pressure build-up is not instant.
Also, if anything, the thermal expansion of the watch should start sooner than heating of the air inside, thus lowering the pressure inside. Of course this would effect would be marginal.
I don't have the wish to do the math on this, but I'm fairly sure my theory applies. My only proof though is general "engineer reasoning".
What are your thoughts?