Correct- OpenROV is inherently very close to neutrally buoyant in fresh water which means it won't sink down or float up if left alone. Because water has (effectively) the same density at any depth, and the amount of water the ROV takes up (it's displacement) stays the same even when under pressure, as long as the mass of the ROV is equal to the mass of the water it displaces, its buoyancy won't change. There is a propeller on the top of the ROV which thrusts it up and down to change depth, and that works great.
As a matter of practice, we also like to make it so that instead of being perfectly neutrally buoyant (so the ROV will stay at whatever depth you put it at indefinitely), we make it slightly positively buoyant, so that if we loose connection with it, it will slowly come to the surface.
Since salt water and fresh water have different densities, (and temperature can also effect water density) the fine tuning of the ROV's mass is done by adding small lead weights (actually little bits of thick solder) to the ROV.
Hope this all answers your question!