Place watermelon in a large bowl. Add basil, blueberries, feta, and lightly toss.
Drizzle lime juice on top. Serve. Leftovers will last in the fridge for 2-3 days.
Watermelon: To choose a ripe watermelon find the big field spot where the watermelon was resting as it grew and look for one with a creamy yellow spot, rather than white, indicating it's ripe. Then knock on it; you should feel a firm thud (not soft and dull), to make sure it's not overripe.