Ardurover Boat - Run aground avoidance

Hello! I previously had a number of boats running prior versions of Ardurover that could use the sonar as a range finder to avoid running aground as described here:

However, 4.0 doesn’t seem to support this; so I’ve tried bendy ruler but cannot seem to get it to work with a downward facing sonar. It does not react to shallower depths. Does anyone have any input? Is this possible or perhaps am I making a configuration mistake somewhere?