I leave mine off. I ran a charter in Nz for 8 years and every time a boat came past with their sounder on the snapper would go off the bite for 10-15 minutes. A good way to stuff someone who anchored too close, put sounder on full and work up their berly trail.
The ping sent down by the transducer affects the lateral line on a lot of fish.
The only time I left mine on was and is when drifting as you are not bombarding the fish with the ping. Try swimming under the boat when the sounder is on. Even thick skinned people can feel the shocks hitting the skin. Let alone a sensitive fish.
Cheers Rowan