Final answer:
The Alpha flag is used to indicate 'Divers in the water' and is part of the International Maritime Signal Flag system, signifying that boats should stay clear and move slowly.
Step-by-step explanation:
The flag that indicates 'Divers in the water' is the Alpha flag (Option B). When you see this flag, it is a signal used widely across the globe to denote that there are divers currently in the vicinity and that boats should keep clear at a safe distance and move at a slow speed. The flag is white and blue and is part of the International Maritime Signal Flag system. It's important for watercraft operators to recognize and respect this flag to ensure the safety of both the divers and those navigating around the dive area.