Another single-track Hakuoki drama CD, this one pitting Otohime against Kazama in what the title calls a decisive marine battle with a revenge of sea creatures. The absurdity of the premise compared to the main game's serious Bakumatsu-era drama is the entire point.
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These bonus CDs let the voice cast play around in scenarios the actual game would never attempt. One track of comedy drama performance. Very 2009 otome game limited edition culture. The title alone, involving sea turtles and undersea revenge, tells you everything about the tone.
If you're deep enough into Hakuoki fandom to care about bonus drama CDs, you already know whether you need this one.