Blue shark

You (or anyone you want to be) were swimming in the blue of this sea. The water was pleasant and clear. Then you dived, enjoying the relaxation, and suddenly you saw a figure writhing beneath you. You were curious, and you're a very brave person (or someone else). It turns out the shark was entangled in fishing nets and was suffering. You took a sharp object and untied the shark. It started circling you, not knowing how to thank you.