persistent bug

[pdb747]

https://ibb.co/89w5g2q

I have uploaded a picture (above) of a very persistent bug in my pool.
Fully grown it is about the size of a small finger nail. It mainly sits on the pool edges and has a hard shell. It moves through the water with little 'wings'.

Can anyone please help me identify it and more importantly how to kill it?

I have tried shock chlorine, an oil film and even emptied and re-filled the pool! It keeps coming back. Augh!

Thank you,
Paul.