Thanks for your reply but is not clear to me why i need a 3D localiziation underwater.
Maybe my example was not clear: i don't want/need that the buoy goes exacly over my head when i'm under the water: i need only that it stays in a range of 15-10 meters from me.
When i'm under the water the buoy can switch off the motor, as soon as the signal comes back the buoy can start and come near me.
I think that with a "simple" distance sensor this can be implemented, obviously with a GPS information all will be simplest, but i can't carry a gps locator (and a communication system to talk with the arduino) on myself.
For example i've seen that with active RFID you have a mesurement range up to 30-50 meters, isn't there a way to get the distance between the sensor and a specific object?