You have to have a unit capable of the output that the transducer can transmit. Maybe the 797 does not. The specs state a depth capabilily of 1,500 feet which is a lot more than the 300'+ you mentioned. Evidently the 83 khz and maybe the 200 khz has the 1,500' depth capability with the power of the 797.
The larger units like the 997 state a depth capability of 1,500 feet and with the 200/50 khz transducer, a depth capability of 3,000 feet. The 1kw transducer is the only one I see listed that has 200/50 khz. The 50 khz is for the deepest capability.