Usually people who want direction under water make a phased array** sonar system. I think a side scan sonar*** is a special case where only one axis of direction is used. If you were thinking above water you probably want to start google'ing for something called sodar instead****.
What I always wanted to try (and never did) was to use a cheap sonar depth gauge, aim it sideways from the boat traveling in a straight line at constant velocity. Then feed the data into a computer to create a something like a "synthetic aperture side scanning sonar".*****