What do you want to achieve? Are you trying to control a BlueROV2 using a Modem-M64?
We have made an example of controlling the BlueROV2 using a joystick directly (Not using QGroundControl).

Code is available on github:

It is not very practical to control the BlueROV2 sending joystick data over an acoustic modem since you don’t have enough bandwidth for sending video back and there is some latency.

A better use of the Modem-M64 would be to send waypoints to an autonomous underwater vehicle (AUV)/autonomous BlueROV2.
As far as I know waypoints are not supported by the ArduSub software at this point. Contact Blue Robotics directly for the status on that support.