When you talk about "rat melon", do you mean "hand melon", which is more famous for the kind of melon used in fish ball soup! ?

If so, the Pearl River Delta area is called "water melon" (you can find it very accurately on Baidu), and Mandarin is called luffa (but there are many varieties of luffa, Baidu found it~~), so if you are in In Guangdong, everyone can understand water melon.