Hi,
Just a minor correction- the T200 is NOT rated to run at 24 V, and is only officially rated for up to 20 V. Running a T200 at over 20 V is unsupported, but you are welcome to try on your own. For a more complete explanation please see my response here:
There is nothing on the BlueROV2 that runs at 12 V, the two UBECs step down from battery voltage to 5 V to power the Pixhawk, camtilt servo, Bar30, Raspberry Pi, and camera. The Fathom-X and ESCs all run at battery voltage (up to 27 and 25.2 V respectively). Its true the UBEC we have been using on the BlueROV2 until recently can in theory handle up to 25.2 V input, but its extremely hard on them. Using another 24-12 V adapter before and feeding this to the UBECs instead can work, but a better solution would be to use a more powerful UBEC on its own that can better handle higher voltages and currents, like the new one we are now including with the BlueROV2.
Out of curiosity, I have tested the Blue ROV2 with a 6S battery (22.2 V nominal, 25.2 V max) with both the old and new UBECs. The new UBECs are definitely recommended if exceeding the BlueROV2 operational voltage like this. All the electronics handled the higher voltage surprisingly well, including the R3 ESCs. However, after extended use at constant full throttle, I was able to overheat and damage a couple thrusters as expected. If you choose to run the BlueROV2 at beyond its operational voltage, it would be best to keep full throttle bursts at 100% gain to short periods at most, or perhaps limit gain altogether to 75% or so.
-Adam