I like my Dive Alert, even used it yesterday. While diving in 60', about 20 miles offshore, we found some interesting bottom to explore, boat watching from above, but not anchored. We traveled at least 1/4 mile away from the boat, when we came up, the boat was out of site, until a swell lifted us up, and the boat could be seen in the distance (very small), even with a radio playing on the boat, they heard the Dive Alert easily, and came over to pick us up, they had not yet seen us, until hearing the Dive Alert, highly recommended, and yes, it is VERY loud, do not point it near someone's ear.
The SubDuck, is made for underwater communication, it's effective, and contributes to underwater noise, but I know some people who like them.