Fruit – What would be a good sub for pineapple, any other fruit or veggie for a strawberry pineapple mint salsa

Adding fruit will change the ratio of dry to liquid, so it may require more flour. It would also add some sweetness, so you may want to reduce the sugar a tad. But citrus fruits in particular are acidic, which might affect the leavening. I suspect it would depend on the type of fruit and how it's prepared.

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.