According to the release notes there was nothing added/changed in the 4.780 software that affects the depth reading programming.
Try this: reset all the menus back to the factory defaults using the Restore Defaults menu. Tan set the Water Type menu to the “Deep (salt)” setting and the Surface Clutter menu to 9.
If that does not help you can always try going back to the 4.730 software. I have it of you would like to try that.