I spawned at a beach and the water was filled with a whole ton of Megalodons there was also spino nearby and killed everything I had so I let it into their territory the spino cleaned them out but then it died.
If you need to do work underwater (I used this when building bridges between all of the islands) then, at least on mobile, you can go into camera mode by swiping from the left side of the screen. THIS PAUSES THE GAME!!! You can use this to check for megs before diving in the water.
I had a megalodon called jaws once, and I put him on wander to mate with another megalodon I had, but I forgot I left him on aggressive and he went after a dodo. He got beached and eventually about 4 dodos ganged up on him and pecked him to death. By the time I was there it was already too late. RIP JAWS ~ death by dodo