Hi @franka,
Center of gravity for a given axis can often be determined practically by balancing the object on two rods/poles, and sliding one towards the other until the object starts to tip. Since the front of the ROV is quite obstruction-free, I’d recommend trying that approach if you want to find the Z axis location of the CoG for your vehicle in a given configuration.
As @SDI mentioned, the center of buoyancy (from the volume distribution) is also important. This thread may be worth a read.