I'm not sure that it would be, considering that DI & SI give the best images when moving forward at a decent speed. They will still produce images when stationary, but will cover only a narrow strip of water.
A portable 2d sonar with a 60deg transducer cone would likely give greater area coverage .... but, you still have to factor in the depth of water under that dock and the cone shape of the sonar beam. A cone angle of 20deg only gives you "bottom" coverage in a circle that's approx. 1/3 as wide as the depth of the water, and a 60deg cone angle gives bottom coverage in a circle that's equal to the water depth ... but, even so, the coverage from the transducer down to the bottom is like this /\ (0 inches to start, then gradually widening with depth).
You would "see" fish movement with any of the three, even with the unit in a stationary position .... but, the fish would have to move thru the beams before any change in the screen image would occur.
This graph is from Humminbird, but gives you a rough idea of what I'm saying :