Snakes are completely other question, though I agree that sea ones have more potent venom than land-dwelling ones. But let's not digress.
No, I didn't read "The Black Pearl". Nor probably intend to.
And I'm not much impressed with anything someone might do on national TV. Expecially at our age of sofisticated montaging.
And that setting aside the fact that most crocodiles aren't very agressive either - they simply don't need as much to eat as mammal carnivores, being cold-blooded as they are.
(Actually, term "agression" is somewhat misplaced here, but that will also lead us far to the side...) And in showbusiness mantas are actually well established afaik - they are quite the looks with their mode of movement, after all.
But this doesn't make them dangerous for those who leave them alone (this will include not to step of them!
). They are slow-moving, for one thing, and their teeth are quite unsuitable for large prey. As for close relations - well, donkey is quite close relative of rhinoceros, and this doesn't make him killer!