Traditionally, SCUBA divers have relied on whistles as their only audible surface signaling device. But, if a diver is disabled and can't blow into the whistle, the whistle makes no sound at all. Equally frustrating and dangerous, if a diver needs to attract the attention of someone who is too far away to the the whistle, the whistle is of no help. Dive Alert PLUS is the only solution that addresses both of those issues. The Dive Alert PLUS is the only TRUE dual function surface/subsurface signaling device.