I think that the angle of the transducer would have to be off quite a bit as the DI sonar has been found to work down to 350 feet in fresh water, I don’t know what its salt water limit is.
Is the transducer mounted on the outside of the hull or is this an inside-the-hull mount?
Check your software version in your 788ci DI unit.
Also see if you have the “Digital Depth Source” menu (you may have to be in the “Advanced” User Mode to see it).