I was bringing back a Yutyrannus I had just tamed back to my house. I went around some rocks and into deep water. There was an alpha whale. The tapejara which I used to get over to the cold got stuck on its tail. Both of them are still alive
It will attacks rafts so be carful coming across one on a boat.
Also the Alpha Albino Leedsichthys Will give you raw prime fish me when you scrape meat off of it, and killing it will give you The Captains Hat Skin