Travel with your newly found raft through the ocean and get to The River, fighting against anyone and everything in your way. They keep coming so do not rest. Fight with increasingly harder vessels and environments, while improving your raft with the flotsam you find. If you thought being alone in the middle of the ocean was difficult, getting near The River is hellish. Don't quit, don't die, and you will hear the Roar of The River soon... Or later. If you can.
An in-game screenshot. The flotsam in front of your vessel are debris from an destroyed enemy vessel. Shadows are OFF.
The ocean and all the materials are using the unity's physics engine.