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This short, tie-less stage offers an achievement/trophy simply for starting it. Figure out the controls for propelling Octodad through the water, and eventually a shark will appear behind Octodad. The method to surviving this stage is simple: just keep swimming. You cannot control the direction that Octodad moves in, and he will escape from the shark through a dark tunnel on his own as long as you don't stop swimming him. Eventually, Octodad will swim into a tunnel where a vent that constantly opens and closes stands in his way. Here, you can charge Octodad's propulsion so that he rushes through the vent as soon as it opens. You can also charge through the subsequent bars blocking Octodad's path, as the force with which he swims will be enough to smash them.
After passing through all obstacles in the tunnel, Octodad will end up in a large tank with some beluga whales. Keep swimming around and Octodad will exit through a tunnel that takes him back to the tank that he started in. This triggers a scene in which Octodad regains his determination, leading into the next stage.