hydrau tech

I am looking for a GPS/GLONASS fishfinder and a transducer. My purpose is a bit on the unorthodox side. I work for a small civil engineering firm that assesses and and designs countermeasures for bridges. Many of our sites are over substantial rivers such as the Rio Grande and Colorado River. The overall goal is to be able to get accurate depth measurements from the transducer that will be logged in a gps unit. The setup will be on a small fishing boat and we will run cross sections upstream and downstream of the structure to get a representation of the channel. We are looking for a transducer with a narrow cone angle in order to get an accurate depth measurement. We also need a gps fishfinder that will log our location and depth as we move through the channel. Any help in selecting the proper equipment would be highly appreciated.