What is the recreational diver flag (red flag with a white diagonal stripe), and what must a skipper do when sighting it in a bay?

1. It marks an underwater fishing competition where all motorboats must accelerate to clear the area
2. It indicates a sunken vessel containing hazardous radioactive waste
3. It indicates an official naval firing zone
4. It indicates recreational divers or snorkelers in the water; skippers must maintain a clearance of at least 50 to 100 meters at slow speed