Mix cubed seedless watermelon, sliced cucumber, and sliced onions in a large mixing bowl.
To make the dressing, stir together lime juice, rice vinegar, fish sauce, grated ginger, brown sugar, salt to taste and freshly ground black pepper in a small mixing bowl.
Pour the dressing over the salad and mix well. Sprinkle roasted peanuts, mint, and basil on top. Serve immediately.