When fighting them for shell fragment farming, jump on the back of them and attack till they hit water surface, then you’ll be able to just attack sitting on top of them while it swims around... side note, if your tamed Dino clips through their model, it has a chance of auto dying... lost my 209 sarco when it was well above half health instantly because it clipped through the model of the turtle
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!