They can only mate in the water. The egg looks like a ball of jelly and floats on the water surface. You can't pick it up! It doesn't spoil either.
This dino is excellent for getting the deep sea loot crates. Be aware that your own stamina runs down very slowly when you're riding.
Amazing for starting out if you wanna go in the ocean and you don't have scuba gear unlocked, they are extremely fast and can outrun pretty much anything in the water like the dolphin, I still use mine even now