So, you want this gadget to notice your head, when you come up for air and you are 20 metres away from it.
That is not a particularly simple problem.
The only way I could see to solve this, would be to attach some kind of light to your head. Could be an IR led, or a laser, or some kind of noisy device.
And then build either a rotating sensor or a conical omnidirectional sensor on the buoy, which is going to detect the light on your head and steer towards it.
This seems not impossible , but rather complicated and tricky.
If I was going to try this, I'd start by getting maybe 20 IR photodetector diodes, and glue them in a circle facing outwards, and then try steering towards the one which is picking up the signal. But getting a range more than about 5 metres from them seems to be problematical.