I just ordered one. The issue is that if the boat is in the distance with the motor running, it may be still very hard to hear.It's a good device, it make a terrible noise when you use it on the surface, you really have to be deaf not to hear it !
(Yeah, I tried it in the dive shop the first time I mounted it on my bcd )
You can have a double one, with an alert for underwater, it makes kind of a "mump mump" noise, it's nicer than the shaker (imho) !