The Scub-Alert is an underwater horn that uses tiny pulses of air – delivered via your inflator hose – to rapidly vibrate a steel disk. When that disk vibrates, it generates a loud "quacking" sound. In this way, the Scub-Alert allows you to signal your dive buddy simply by pressing a button. Or call some ducks. Considering the range of the horn on the surface is 0.25 to 0.5 mile, you’d be wise to note the warning: “Horn is very loud above water. Keep away from ears at all times.” You can have your own ear-piercing horn for around $55.

The same company also manufactures the Inflat-Alert, which combines the power inflator and the Scub-Alert into a single unit. Using this combination device eliminates the need for an extra length of hose that the Scub-Alert adds to your L/P inflator configuration. 

Related: 

Buzz Your Buddy With A ScuBuzz 

Underwater Noisemakers