@sigsgmret this feels like one of those events when "perfection" is the enemy of good.
What level of perfection are you seeking? A depth sounder is most often used on a sail boat to return data that keeps the boat floating and not stuck. If the water depth is 5 feet. Your keel is 3 ft and the depth sounder reports to you there is 1 foot of water beneath the keel then you are still floating. If the bottom is typical there are ripples of 6-8 inches in the sand. You are still floating because you are using good not perfection.
So perhaps setting the sounder to 3 feet or even 4 feet.
In most sounders the issue of Deadrise is managed by adding a wedge to the sounder where it connects to the hull. The wedge corrects for the angle of the hull at the location used to install the sounder so that the sounder transmits a vertical signal into the water. Note sounders send a signal as a cone beneath the boat. If it is angled it will send a response back to the transducer giving you the distance at that angle to what ever it bounced off.