With reference to this issue on github:
Rover: cycle through rangefinders when sending WATER_DEPTH by peterbarker · Pull Request #28877 · ArduPilot/ardupilot · GitHub
Is the fix (per December 24) included in the 4.6.0 beta ?
I installed the beta4 yesterday, and I see that the WATER_DEPTH message is still arriving at 50 Hz which is a real problem, and the data rate of the transmission I use increases 3-4 fold as soon as serial data from the transducer is detected by the autopilot. Resulting in occasional heartbeat timeouts.