The Devonian Period is a world in the Prehistoric Universe mod. It is completely water apart from some islands.
Most of the world is an ocean which means you must have already crafted a scuba Armour (At least a helmet). The first thing you'll notice is the amount of little fish. If you get deeper you'll notice the size difference. Once night falls, Dunkleosteus and Helicoprion go to hunt. If you're in luck you might find an island which there will be groups of Acanthostega. These will spawn eggs. There you can take them back and create your very own Acanthostega babies.