A tremendous amount depends on what the fracture was and how the pieces were oriented. "Ankle fracture" can range from a small avulsion off the distal fibula as a result of a bad sprain, to a tibial plafond fracture that can result in ankle fusion. None of us can tell you, without a great deal more information, what the prognosis is for being able to weight bear and kick by the end of September. If you are fairly young, and the fracture was a nondisplaced fibular fracture, you might be just fine. If it was a more complex fracture, you might have trouble.
The person to ask is the orthopedist you are seeing for this injury. Describe the physical demands of scuba to him -- the weight you carry and the mechanics of kicking -- and let him decide whether it is appropriate or not.