In a large bowl, combine the cubed watermelon, sliced red onion, chopped mint, and chopped cilantro. Gently toss to mix the ingredients evenly.
In a small bowl, whisk together the fish sauce, lime juice, and sugar until the sugar is dissolved. Pour this dressing over the watermelon mixture and toss gently to coat.
In a skillet, heat the vegetable oil over medium heat. Add the sliced shallots and fry until golden brown and crispy, about 3-5 minutes. Remove the shallots with a slotted spoon and drain on paper towels. Season with a pinch of salt.
Serve the watermelon salad in bowls, topped with the crispy fried shallots. Adjust seasoning with salt and pepper as needed.