Megalodon have quite a big turning circle. Pump movement to at least 120% and go find one using a scuba suit, get next to it but a bit towards it's tail end, shoot with tranq arrows and it will circle round and round until its KO'd. Enjoy your new pet!
1. Get an ichthy with a saddle
2. Find a megalodon underwater
3. Kite the megalodon to shallow water with your ichthy
4. Tranquilize it from the shore or a rock safely
5. Make sure you don't keep shooting it after it is KO (I learned this the hard way)
6. Name it Bruce