Several possibilities:

Cranking battery is old, or is too small for your outboard. The fix is a new battery with sufficient CCA for the outboard.

Cranking battery isn't being charged fully between trips or is being used heavily without enough outboard run time to recharge it during the day. The fix is eitherchanging charging practices, a new cranking battery with larger RC (keeping the CCA requirements in mind) or spending more time running around and less time fishing

Bad connections in the cabling to the 798. If you are tapped in at a panel, try running the wires directly to the battery.

Another solution is to add another battery dedicated to the electronics, along with a charger.