In a small bowl combine onion and vinegar. Marinate covered in refrigerator for several hours or overnight (I did overnight).
Cut avacados in 1 inch chunks and toss with lime juice. Cut watermelon into 1 inch chunks.
Toss together watermelon, avacado, orange juice, olive oil and cilantro. Drain onion and add to salad. Add salt and pepper. Garnish with cilantro.
The recipe said 2 avocados and 6 pounds of watermelon...I told you what I did above.