Welcome to the Humminbird Side Imaging Forums Beaulieu80.
Did you put the black rubber plug into the Compact Side Imaging (CSi) transducer when you installed it?
Where do you have this transducer installed versus where the 565’s transducer was installed?
Can you post pictures of the transducer install on your boat? We would need to see an overall picture of the transom of the boat plus several from both sides and closer up and all at the level of the transducer.
When the 798 unit was showing the incorrect depth, were there any other sonar units running at the same time? Does it do this with all other electronic devices shut off (including the motor)?
Try adjusting the Surface Clutter menu to a lower number, possibly all the way down to a “1” setting. If that does not do enough try setting the Water Type menu to the “Salt (deep)” setting.