You should be interested in decibels. You can only define effective distance by defining wind speed and direction, surf noise, engine noise, ambient noise from birds and other masking noises that may be present. The range of the dive alert is perhaps up to a mile or as short as 100 meters depending on these factors. I have gotten the boat's attention from several hundred meters in "average" conditions.
The dive alert is far more effective than a whistle. Do not make the mistake of activating it with your head above water. Hold it up aimed at the boat, lower your head under water and activate. Always warn others in the area to protect their hearing. This is not a trivial warning.