I'm thinking more of a busy shipping area, and the essential mission is only to provide a surface exclusion zone, which a submarine could take advantage of as it (say) enters / leaves a harbor. The sonar's job is to broadcast a "your (say) Coast Guard Auxiliary moving exclusion zone is right here" signal to the sub's passive listening equipment. The CG Auxiliary guys up top don't even know if a real submarine is in the area - vs. a practice mission, or maybe the Navy is sailing a cheap decoy past some Russian fishing boat that's lingering where it ain't welcome.

(From exDM69's comment, I'll figure that the active sonar's power needs to be dialed way down. The theory that a sub's fancy listening gear could get a good fix on a sonar signal that's too weak to give a usable reflection seems hand-wave plausible to a landlubber.)