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.
This guys are a bit of a pest, all the more while taming in the open waters. Use a strong melee weapon to slay it fast. A good enough sword can be obtained after completing the Hunt: Terror Birds pursuit.
Use a high level Basilosaurus with high health to lead it to shallow water where the basilo gains health faster. Dismount and let the megalodon bite at your basilo while you traq it. Tamed 4 150 megalodon with this method works like a charm.