When taming, make a box using gates as far into the water as you can (but still a bit on shore). Use foundations and door frames on the open end facing the shore to make the space for the manta to beach itself on smaller (make sure the door frames are on the inside of the foundations so it can’t splat onto them)
They have terrible turning circle, approach one from above and feed it. Then let it swim circles around you. The occasional time it gets close you can predict its attacks, it “bucks” just before the attack, which is the tail swinging underneath. Quick swim up or back avoids it. Be prepared to get dizzy, long time between feeding
Tame a kaprosuchos and swim to one of these bad boys and do it's alternate attack it will grab it in it's mouth. walk onto the shore where its safe and have another person feed it while in the gators mouth (if you unmount it drops it) it wont attack or die because it's technically not on the shore.