She was sharing her experience about her whole family being on a fruitarian diet with the exception hot cereals in the morning. Someone asked her how she managed that diet during her pregnancy and infant care. She said that watermelon was what got her breast milk flowing.
Apparently, it is good to take in more fluids. Watermelon has a lot of water. So, I think more liquid containing foods, like soup and watermelon could help.
It may not be the water. Watermelon is very high in potassium, which is one of the reasons it is so diuretic. Breast milk is also very rich in potassium so it could have something to do with the potassium increasing milk production. Interestingly, fenugreek that also increases breast milk contains around 28mg of potassium per teaspoon of seed. That would be considered a rich potassium source.