While they have a lot of health they are stupidly usless to fight back attackers, needless to say overrun them; Specially in water where every 10 feet theres 3 f*cking eels, 5 sharks and 2 mantas comming atcha.
Avoid running into fights as much as you can or always have a couple badass tames or tribemates following you to clean up attackers, otherwise you will be struggling for your tame's life as well as everything you brought with it.
Probably the best platform dino to live on, the megachelon can fit a 5 by 7 of foundations on its shell, and many structures, like chemistry benches, grinders, and even industrial forges can be built on it while staying there when underwater. You don't even need scuba gear when going underwater, staying above one provides oxygen. Not only this, but they actually have surprisingly good melee damage, as well as being one of, if not the best fiber harvester in the game. Different resources will constantly build up in its inventory, including rare mushrooms and flowers. They also automatically irrigate any crops placed of them. Get an imprinted one for extra speed, damage, and resistance, then give it an good armor saddle from the frog escort mission in the bog, and you have an extremely tanky resource gathering portable base, war machine, greenhouse, and water exploration mount!