Add orzo to a medium saucepan. Place saucepan over medium heat and toast orzo, tossing pan occasionally, until orzo is light golden brown and fragrant, 4–5 minutes.
Carefully add water to saucepan to cover orzo by an inch or so. Bring to a boil, then season heavily with salt.
Cook orzo until al dente according to package directions. Drain pasta then return to saucepan and toss with a glug of olive oil to keep pasta from sticking; set aside to cool.
In a large bowl whisk together 3 tablespoons oil, 2 tablespoons vinegar, ½ teaspoon salt and ¼ teaspoon pepper.
Add cooked and cooled orzo, watermelon and tomatoes and toss to combine.
Stir in basil and pecorino. Season with additional vinegar, salt and pepper to taste.