That buoy is achored or could you pull it towards your position?
If its achored you could just use some wire and a gyro/accelerometer and keep the wire/cable under some force.
With the buoy position, wire lenght(like those dog wires) and angle you could determine your position.
But the waves and the motion in the wire will add many problems for a measurement. Maby a second gyro just to caculate the waves 
If your buoy is not anchored it will just move towards you. Maby you could pull the wire and wait untill the buoy is right above you and pull a trigger to save that position / wire lenght of the buoy.
But that could be pretty slow
But now you could also add like in the flying examples swimming ardboat-buoy´s that keep their position and measure like said above.
Underwater IR is absorbed really quick. Ultrasonic will irritate animals i read somewhere.
All other options i think require more than one transmitter/reciver or could irritate animals like electric fields from 2-3 buoy´s ...
There is a underwater navigation called tamin i found.. but seems expensive 