Glad to hear it 
The center of mass is determined by the mass of all the components - without knowing the mass and location of each component in your ROV design it’s not possible to say for sure where the center of mass is. As a trivial example, you could have a very heavy weight at the back of your ROV, in which case the CoM would be much closer to the rear than the middle or front.
If the rear thrusters are in fact further from the CoM than the front thrusters, the torques generated by the thrust force from the back will be larger. That shouldn’t affect moving forwards/backwards, but when trying to move directly sideways the ROV will tend to rotate by leading with the back, and if you’re trying to turn the back will turn a bit faster than the front.
I don’t imagine that will cause many issues in a tunnel inspection, but it’s at least worth knowing about