I doubt you’ve unknowingly damaged the tether, but might be worth making sure the connections are all physically stable (e.g. Ethernet cables fully plugged in, fathom X wires screwed in properly, etc) for the full set of connections between the RPi and the surface computer (RPI → Ethernet cable → fathom-X → twisted pair → tether → binder plug → FXTI twisted pair → fathom-X → Ethernet to USB converter → USB-A to B converter → USB B to A cable → computer)
The main other thing that can be tried is swapping to a different twisted pair in your tether if you’ve got a spare one, in case the original one is damaged/corroded, but it’s pretty rare that that’s the problem.
This is another thing that could do with some clearer documentation 
The different services have names after the screen -dm -S, which are hopefully relatively intuitive for the most part. I’m meant to be sleeping, so trying to give enough detail without taking too long:
- pingproxy allows ping1D to connect
- pingmav allows ping1D to send distance measurements to QGC
- mavproxy connects QGC to the Pixhawk
- video is video stream
- webui is the companion web interface
- webterminal is the terminal you’re using (probably good to keep, but not essential if you know how to use ssh, user:
pi, pass:companion) - commrouter is for forwarding connections, at the /routing page of the web ui (not essential if you’re not using any such connections)
- audio is saved as part of video recordings, although QGC doesn’t play it live at the moment
- file-manager allows you to get files off the companion computer (can comment if not using, also possible to do directly with scp instead)
- nmearx is for NMEA GPS (can comment if not using)
- wldriver is for water linked underwater GPS (can comment)
- bridgemanager enables ping360 connection
- mavlink2rest provides a mavlink terminal in the webui (can comment out, always possible to restore later if you for some reason need it for something)
- network-service I think handles wifi connections, probably leave it
Yeah, don’t disable mavproxy, although as an interim measure you can try reducing the --streamrate parameter at http://192.168.2.2:2770/mavproxy, which may help with CPU usage as well.
Hope that’s helpful, and really hope it’s not too late for your weekend fun!
Edit: if it’s helpful, I believe the biggest differences would be from reducing the mavproxy streamrate and commenting out mavlink2rest - the others you can comment if you want but they’re unlikely to change much (they do very little when they’re just waiting for a device to connect).
Edit 2: .companion.rc is a startup file so changes won’t apply until you reboot the RPi through the web interface or power cycle the ROV. Can’t remember if mavproxy settings updates are instant or also require a reboot