This stereotype is 100% confined to american culture and its surroundings. Outside of america this is just not a thing. So if you aren't even aware of it, making the connection between black people and watermelon doesn't even cross your mind. The whole world doesn't revolve around the US.
Educate them and save this condescending attitude for something else.
For all the inclusivity talk that goes on here, posters here often forget how USA-centric this board is and assume American culture is the culture of the entire world. Very imperialistic.
For example in my country the racist black caricature "blackface" is a thing, but watermelon is not a thing.