Agreed, 14.3V for a blue robotics battery should be fine.
Are you able to turn the propeller with your hand while the ROV is off? If not, it’s jammed and probably has something stuck in it (e.g. fishing line), if you can turn it then you have a broken connection between the thruster and its ESC or between the ESC and the Pixhawk, or your thrusters aren’t set up properly for your selected frame configuration.
Did you arm it while in depth hold mode? If so that’s a known occasional issue that we’re waiting on some logs for so we can confirm what’s going wrong. I’d suggest arming in manual mode and then turning on depth hold mode when you want to start maintaining a depth below the surface.
Temperature (1) variable is the Pixhawk temperature.
Temperature (2) is the external temperature measured by the Bar30, assuming you have a standard setup.
The companion (Raspberry Pi) temperature isn’t currently accessible from QGC - it needs to be accessed via the System page of the Companion Web Interface.
You can use an aluminium enclosure and/or install a fan in the enclosure to improve cooling, but it can also be helpful to avoid things getting hot in the first place by not running the thrusters at high throttle for extended periods (because the ESCs heat up), and reducing Raspberry Pi CPU load where possible (are you on the latest companion software?)
You mentioned earlier that you replaced the twisted pair from the tether to the Fathom-X. Did you also change the pair in the FXTI at the top? Only the blue-white twisted pair is connected by default (the other pairs are intended as expansion options for other purposes), so if you change the wires at the bottom but not the top then you’re relying entirely on parasitic capacitance along the tether for your signal transfer, which is understandably not as effective as having a wired connection.
If you only changed the bottom ones then I’d suggest you try changing that back to the blue-white pair, and running a network test of the tether connection. If the upload and download speed are both above 50 then your connection is fine, and if they’re over 20/30 then it should at least be workable. It may be worth doing a comparison with the direct network cable connection to see what kind of values you get with a short and very stable connection.