First off, get a whole bunch of these. You get your mantas or itchy(if you're on mobile), swim into an army of beasts, attract all of them into shallow water, and use your spyglass to see which ones you want. Then kill the ones you don't want with a good Dino or weapon(I used dire bear). After that block them off from the sea so that they can't run away and yeet them with tranques. They are unconscious for a super long time so don't worry about constantly hovering over the,.you don't need a pen or anything just make sure nothing interferes. Then you got yourself like 10 Megalodons.
They seem intimidating, but they're surprisingly an easy tame. Personally what always worked best for me was attracting them to shore, placing a bunch of dead dodos on the shoreline. The shark would get stuck in the ground, then I would use a long rifle with tranq darts and shoot it in the head.
When I went to the red obelisk it was by mistake and I didn’t even realise the Megalodon army I just went through the water on my raft and went to see what it was. While I was doing that I had a mission to go the whole way around the island and I left the raft which reset it and I was 4/5 of the way there.
These terrors of the ocean may seem a very strong and a late-game tame, but my first megalodon was tamed when I was less than level 20, all I did was go into the water and lure it to a place shallow enough then knocked it with a club, took very long to knock out but it was worth that time.