There must also be a hydrophone, or something that works like one in the black cylinder part of it.
Yes, the "$35 fishfinder" transducer is pretty solidly constructed and heavy. It must be submerged by at least 20 cm, and comes with a foam float and cable stopper to set the distance.
Battery power is 4xAAA and the lifetime is only 4 hours of sonar readings (3 per second or 1 per 3 seconds in battery saver mode), so the unit must output quite a bit of acoustic power.
Will get a chance to try out the "fishfinder" this weekend in ocean water, then it comes apart for reverse engineering.