How does pineapple react with water?
I have never heard the water-pineapple thing. It may be a confused result of the fact that fresh pineapple cannot be put in a gelatin because there is an enzyme in pineapple (and papaya) that breaks down the structure of the gelatin and it will never set. Canned pineapple is fine to put in gelatins though, the canning process breaks down the enzyme. I have definitely gotten water on pineapple (in fruit salads from other fruits) though, and I didn't explode... It might also just be that pineapple doesn't need or benefit from washing and it turned into 'NEVER GET IT WET' at some point.