A mermaid

You were honestly half surprised when you felt a tug suggesting that you'd managed to catch something in the net, and you were more surprised when it jerked hard, almost pulling you out of the boat.

You braced yourself against the side of the boat and started pulling back. Whatever you had caught, it must have beeen a big one.

Finally, with a mighty heave, you pulled the net into the boat, stumbling back as it came over the side, then falling onto the seat in shock when you saw the contents of your net.

From the waist up, she was almost human, except the ridges on the side of her neck and her sharp triangular teeth, which were plenty visible as she snarled at you. She wasn't wearing much, just a necklace of stones and small shells and an actual clam shell bikini top like a cartoon, which covered her below average but not insignificant breasts.

From the waist down, she was a fish. A big fish by all means, with sea green scales that seemed to turn blue or even purple when the light hit them right.

If you had been surprised at catching something, that was nothing compared to your surprise at having caught an actual live mermaid.