Mosasaurus: I am the strongest water creature of all time
Leedsichthys: Hold my raft chunks

Despite it's dossier stating that it is not tamable it has a saddle design in the ARK Files.

If near the shore, can be used to get some prime fish to help a tame or meat to spoil in early game. Can be quite helpful

If your raft gets hit once, stop it right away. 50% chance it won't hit it a second time and you might just save your raft, simply wait for it to go away then sail to shallow waters asap. You cannot outrun it with a raft and our only takes 2 strikes to sink a full health raft.