It's the Law: Required Equipment for Your Vessel

Diver Down Flags (625 ILCS 45/5-19)

Persons scuba diving, skin diving, snorkeling, or underwater spearfishing must display a diver down flag to warn other boaters. A diver must stay within 150 feet of the flag. Unless there is an emergency, a diver may not surface more than 50 feet from the flag. It is unlawful to display a diver down flag when not diving. Vessels not engaged in diving operations must stay at least 150 feet from a displayed diver down flag.

The diver down flags are:

Divers Flag: A rectangular red flag, at least 12 x 15 inches wide, with a white diagonal stripe
Alfa Flag: A blue and white International Code Flag A (or Alfa flag), required on federal waters