The only accurate way to do it is with the 2D sonar.
Since in SI/DI, you can't easily determine the real distance between the structure and the boat.
50 feet in Si range, is calculated from the transducer to the structure, in linear fashion, like a laser
beam. Not parallel to the ground. So the real distance can be 45-48-40 feet, the deeper, the more
prone to error your measurement would be!