Ha Ha Ha ! You guys are all too young. It's a variation on the famous (or infamous) 1949 Plymouth 6-cylinder distributor syndrome.
My brother, five years older than me, had one when I was 12 years old. That was 57 years ago. He had to do the same thing every time he started it. Except that if he didn't do it, his distributor cap blew off explosively. For him it wasn't water vapor, it was fuel vapor coming up past the distributor seals from the crankcase, which ignited when the points sparked the first time. I suspect that if you pull the distributor and disassemble it, you will find the seals between the the distributor shaft and it's housing are damaged or worn. I don't know what the inside of this distributor looks like, but I've never seen a distributor that didn't have a shaft seals.
There are always three things in the crankcase of an engine whose oil hasn't been changed recently - oil, water and fuel. When the oil gets hot, the fuel and water vaporize and if the distributor seals are bad, it comes up around the shaft and condenses when the engine is shut off and cools down. The fuel sometimes ignites on cold start and sometimes the water vapor condenses in the cap and keeps it from starting. This is just a guess, based on 57 year old experience, but it's also the only way that I can see water getting into the distributor cap.
I know someone here has experience disassembling and rebuilding Bosch distributors. Hopefully they will speak up. Dan? Joe? Anybody else?
Good Luck.
Tom Kizer