Hi @lunzzii,
QGroundControl and BlueOS both provide options for flashing Pixhawk flight controllers, although QGC has a few more options available. It’s best to install a firmware for the vehicle type you’re intending to use (e.g. ArduSub for ROVs, as @tony-white mentioned), and I’d recommend starting out with a stable release to make sure it’s working as expected.
Please also note that the Pixhawk 2.4.8 has been known to be unreliable, so it’s more likely to be a hardware issue than with other flight controller boards.