Just an update to everyone who has come across this thread hoping to reduce the load that MAV communication has on their companion computer: Mavlink-router can not be installed on the BlueROV companion because the Python version on Raspbian Jessie is too old and can not be upgraded. An alternative that we got to run successfully is mavp2p. It comes pre-compiled for Arm7 architecture. Download and unpack on your Linux-laptop, then upload the unpacked file to the companion’s home directory using the file manager. Then go into the Pi’s terminal and run:
chmod +x ./mavp2p
screen -X -S mavproxy quit
./mavp2p serial:/dev/autopilot:115200 udps:localhost:9000 udps:0.0.0.0:14660 udpb:192.168.2.255:14550
First line makes the script executable (this only needs to be done once after the file upload), second line kills the running mavproxy service (QGC will notice and tell you that it has lost communication) and third line restarts mavp2p with the same settings that were used in the mavproxy default parameter file (QGC will tell you that is has regained communication). If you made any changes to your mavproxy parameter file, you will need to change the arguments accordingly.
Finally: Since the new BlueOS can be run on more versions of Raspberry Pi, things are likely to be easier in the future.