for example, with my customers, I'll use a different audible signal to, say...."look, here's a nice nudibranch" from "watch out for that submarine" or "you're on your way to the bottom 80m/270' away, ascend a little!" With a competent buddy or customers, they often need only a hand signal or a little underwater verbal signal to get their attention. If it's a more serious situation, or a "time dependent" event...such as a school of eagle rays cruising by while some of the dives are staring at clownfish or whatever...I keep a big metal washer, attached by a line, in my pocket...pull that out & tap on the tank. The more I dive, the less likely I am to use something to make noise underwater....I'm in less of a hurry & if it takes a while for a diver to notice, that's ok...unless it's an emergency, of course.
It's frustrating when some guides use a hammerhead (air horn) or whatever every time they see a fish, or a diver gets a bit farther away than they'd like, etc. There is actually a guide here and I "know" when he's in the water, even if I can't see him....because he uses devices such as this soooooo often.