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.
Not even worth trying scary do not think it doesn’t see you I was just wondering through the water trying to get to one island to another I always check the water before I swim once i make it I chase a titanosaur and boom a megalodon shows up do not swim