I haven't heard anything about watermelons or bananas but would avoid large quantities of pawpaw/papaya (small bits are ok). Large doses of the enzyme papain can cause uterine contractions and even spontaneous abortion (we're talking LOTS of pawpaw here mind you) - its a polynesian folk 'remedy' for an unwanted pregnancy. Don't stress if you've had a little bit in a fruit salad or smoothie but I wouldn't eat a whole papaya IYKWIM