I'm not sure if this was a coincidence or not, but when trying to kill one (after the mantas), a swarm of eels came to fight me. It was my first time encountering eels and almost cost me a high level megalodon. Proceed with caution.
On The Island, I always find very quickly in the waters near South Beach, around 65 longitude. They're generally not difficult to spot, due to the swarms of Mantas and Ichthyosaurs. Though it's only happened to me once, I've found one stuck between a large rock and the beach itself, at the south edge of the rocky penninsula near 50 longitude.
Extremely op I 1v2d a lvl 125 and 95 squid with a lvl 15 raw meat tame whale. After a long fight I killed both easily. I recommend putting in health stam and melee so that u can stay under longer, stand in fights longer, and hit harder