Same raise time as a Rex! Difficult part is breeding these things. You have to breed them on genesis in a tek cube at 20, 50 inside the pool in the cube. After that, the egg only incubates in the water at the bottom around 32, 52. I recommend building an egg house on the back of one of your adults and using it to protect the eggs. Never run out of oxygen. Be aware of mosas, eels, sharks, swarms, and jellies. Otherwise, easy raise. Holds berries super fast!
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!