I was riding on my Sarco in the ocean coming back to my base from Herbivore Island and I decided to dive down to the bottom of the ocean quickly because I can. I almost got to the bottom when I saw a Mosasaurus swimming around and I freaked out and went right back to the surface of the ocean and swore not to go back down there ever again because I don’t wanna die.
A good way of looking for these things (mobile): sweep the border of the map and the ocean floor, using an ichthy to keep everything away. Once you find one, park your ichthy somewhere, then swim up to the depth limit of the mosa (you might have to test where it is though, be careful, mosas can swim up FAST). Being off the ichthy will catch its attention. Swimming up to the depth limit will cause it to Aggro off of you, so go back and forth with going to its depth limit, attracting it, and swimming back up to fill it with tranqs. The basic idea is attract, swim up, fill with tranqs, repeat. Hope this helps!