Heat oil in a saute pan and lightly brown the onion over medium-high heat.
Stir in the bell pepper, celery, cumin, chile powder, and cayanne, and saute for 1 minute.
Transfer to a stainless steel bowl and season with salt & pepper.
Add the cooked beans and the remaining ingredients; toss well to combine.
Chill.
If you are preparing the salsa ahead of time, add the mango and watermelon just before serving.