I was in the middle of the open ocean the first time I encountered these. One started chomping on my raft as I was going to the Herbivore Island. I had no clue what it was so I just kept plowing through the water. In the end, I made it to the island and the Leedsichthys didn't destroy the raft entirely. I hopped in the water to get a look at it and it just sat there. They don't mind people, only boats.