For anyone who has trouble finding these or any other deep sea species on mobile, they usually don’t load until you reach the floor so if you use a species that is quite slow which would cause you to drown without special gear or can’t go very deep because they’ll take pressure damage like basilosaurus you’ll likely never see them. the best thing to do would be to use an ichtyosaurus as they are fast enough to allow you to dive all the way down, explore for maybe 5 seconds then reach the surface in time without using gear plus they are very easy to tame (though they have lost their ability to not get aggroed by anything so be careful)