In real life these were vicious carnivores that would have killed and eaten a human on sight and lived mostly in shallow waters and swam up rivers looking for food they were also twenty metres long, good thing the developers didn't make them more realistic or crossing rivers would be a dream until you have a flyer.
A balisosaurus will always be surrounded by some mantas(I haven’t actually counted how many, but it is a decent amount), aka. The death pancakes. If you are swimming nearby/attack the balisosaurus, the mantas will attack you. However, if you best the mantas and the giant whale, you can get a lot of oil and hide(with blubber as an exclusive win if you choose).