Many things happen - 1. Everyone knows that the inside of a watermelon is usually cooler than the hard shell when kept outside. read more
When you soak the whole melon in water, the shell slowly gets cooler gradually, and bit by bit, over the hours, the cool effect seeps inside. And it makes it sweeter. The skin is usually coated with insecticides, pesticides etc. Soaking in water helps remove them and detoxifies it. read more