Thanks Phedias for your question ans Rusty for your answers.
"...Raspberry Pi to bridge between Ethernet and the Pixhawk serial port." => So we need a serial connection between Pixhawk and the RPi.
then "...An Ethernet based tether (like our Fathom-X or -E tether interfaces) allows you to have a Ethernet network connection between your ROV and the surface...."
I don't understand what you mean. As far as I know that there exist an "Ethernet" port in RPi, why don't we connect the cable (cat 5) directly to RPi in one side, and in other side connect a/directly (cross cable) ; b/via a switch, then from an other cable from the switch; to the Ground station computer (with installed QGC)? Do you mean to use the Fathom-X in this case as a switch, that can help to extend the distance of the tether?
Other questions, relating the attached schema. If anyone has experience with Xbox 360 controller (joystick), connected wireless with the ground station computer (with QGC), used for manually control the Bluerov?