I'm not sure but they sure seem to be good at it. I vaguely remember hearing about them from a different case.
I looked around and saw an article from May where it described it started as environmental cleaning up waterways but then discovered a vehicle one time, then a person who owned a towing company joined and it switched to focusing on pulling vehicles mostly out and then one time they found remains inside a vehicle so it sounds like that was the catalyst to the missing person and cold case stuff.
It really sounds like reading that article from May it's not sonar equipment itself necessarily rather "many police departments may have sonar equipment but may not understand all the technicalities that go along with it." And they go on to say ""There’s no school, really that exists, that teaches sonar," Bishop said, adding that he and the team have had to learn the equipment as they go on missions." And I don't think they had the most amazing sonar equipment in the beginning when it was just environmental pulling vehicles out of the water. I'm sure they upgraded over time.
So self-taught specialized learning as they go apparently is a crucial part to how they are finding people. Sounds like getting better each time perhaps?