How to make something neutrally buoyant?
Figure out the weight of the device (ROV, submarine,etc), and
then the weight of the water displaced. When they are equal, the
device is neutrally buoyant. However, usually engineers create a
ROV with a slightly positive or negative bouyancy, personally I
perfer a positivly buoyant ROV because if you lose power or the
tether is cut the ROV will ascend back up to the top.
A more simple answer is that when the gravity & buoyancy are
equal it is neutrally buoyant