Cut each watermelon round into 4 pieces, for a total of 8 pieces*. Brush each side of the pieces with olive oil and a little of the lime juice, reserving at least half of the lime juice for after cooking. Season with a little bit of salt (about 1/2 tsp total for 8 pieces, both sides).
Heat an outdoor gas grill to high heat. Grill watermelon for 3 to 5 minutes on each side, allowing grill marks to form.
Remove from grill and season with the remaining lime juice and sprinkle with cilantro. Season with additional salt to taste. Salt brings out additional flavor so really do it to taste.