The Storm Safety Whistle is not great just for divers but for anyone who needs to be heard in an emergency. Features of the Storm Whistle include: Can be heard underwater up to 50ft Unique patented design allows it to be heard even under water Sounding chamber forces all water to be purged when the whistle is blown The Storm whistle is twice as loud as any other mouth-blown whistle in the world Survival bright yellow color, orange, or black Creates a clear, high frequency sound, with a power rating over 75% greater than other referee and safety whistles The Storm All-Weather Safety Whistle is the loudest whistle in the world It&';s perfect for divers, lifeguards, SAR, and for recreation Creates a clear, high frequency whistle sound, with a power rating over 75% greater than other safety whistles or referee whistles Dual chamber quickly purges for uninterrupted sound High-impact thermoplastic Even for someone who is short of breath. Responsive - Emits the loudest sound with minimum effort Adopted by the US Military, police and security forces worldwide! Be safe, be found. U.S. Military approved. The Storm Whistle is a must have for any emergency situation when you need to be heard.