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?
Not easily. Sonar works to get distance under water but radio does not. But how, exactly, would sonar work? If the buoy had a sonar transmitter, how would it distinguish the echo coming from a diver, from any other echo? If the diver transmits a "ping", how would the buoy determine the travel time? Even if the buoy could somehow measure the distance to a diver, it would in addition have to determine which direction to travel in order to keep within a certain range.
The more you think about the problem, the more you will realize how difficult it is to solve.