First, make sure that you have tried both the maximum and minimum settings for the Surface Clutter menu on your 798ci Si unit.
Next I would try unplugging the AS-Si-DB-Y cable and test both the inside-the-hull and Side Imaging transducers by themselves to see if you get the correct water depth readings. This will tell us whether the problem is in the unit, transducer splitter cable or the inside-the-hull transducer.