In the case of the Plesiosaur-shaped creature off Cornwall related in Tim Dinsdale's book Monster Hunt (above), the creature was also seen fighting seals from above, and in a similar tradition relating to the Icelandic Skrimsl, the monster was characteristically said to decapitate seals. Putting these observations together, it seems that Longnecks can feel antagonistic (territorial? defensive?) against seals and sea lions, and it deals with them by throwing them up into the air to break their necks.
Bonus: chart of the seals, sea lions and walruses of the world for reference.