" If a depth sounder isn't Sonar what is it ? " - well there are various types of sounder, some like mine just work straight down like a Radalt / WW2 Asdic, while fishfinders, forward looking Sonar ( FLS, to try to avoid steering into shallows or cliffs ) and true SONAR will provide a wide scan undersea charting capability.
BTW my feeble little depth sounder was once good enough to detect a sub' trying to hide underneath us in a NATO exercise - we were motoring in a calm, mid English Channel, the ships and helo's had detected him alright and dropped smoke floats, but were kind enough not to use depth charges or torpedos ! I cottoned on, and our depth reading went from 60 fathoms ( old money ) to 20 as the crafty git tried to hide under our diesel sound...
I'd rather have Dolphins in company any day - though I did once have a girlfriend who was in danger of being chased by Japanese fishermen.