What you want to do is twofold: first 3D localization underwater, then have the buoy steer to a place above the desired position. Given a target position, the steering part is not particularly difficult, but if you browse through this and other forums (or google "3D indoor localization") you will find that the localization problem comes up a lot. It is very difficult. Given a limited budget, by far the best solution to date on land is GPS, but that requires a clear view of the sky and won't work underwater.
I don't like to be negative but in my opinion, what you want to do is just not possible with current technology.