I believe it is "Indian cucumber" also known as "Yellow Cucumber" or "Vellarikka".

It's not a watermelon. How big are the leaves? How big is the fruit? What does it look like inside? What are the seeds like? I don't think it's either of the following, but it has similarities: Cucurbita ficifolia and a Thai melon (C. Melo). C. Ficifolia should grow to the size of a spaghetti squash, and feel strangely smooth; it keeps for a really long time, has white flesh, and black seeds when fully ripe; it's also stringy like a spaghetti squash when fully ripe.