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.
*This is one of those recipes where it a written recipe with exact amounts is not really necessary. You can make as much or as little as you want and easily adjust it. Slice the watermelon 2 inches thick. The above written list of ingredients can be used to estimate how much you'll need for either more or less.