I’m a Japanese native and own the series. Being Japanese, it is rather difficult to provide you with an exact suggestion of the title in English, but perhaps I can help you by giving what sort of nuance I get from the Japanese title ‘滄海よ眠れ’.
First of all, this is certainly not a happy or peaceful title – Japan lost the War, and many Japanese still share extreme sadness and regret about losing so many lives in the war. And perhaps this emotion extends not only to the Japanese who fought in the war but also to the war-time enemy – the Americans. So ‘滄海’ here is definitely NOT talking about the sea itself, but the vast number of war victims, primarily who died in the ocean.
‘滄海’ implies the great ocean (refer to 広辞苑 if you have the chance), which in turn implies the greatness of the value of so many lost lives. So this title expresses a sort of mourning for the dead – both the Japanese and the Americans who lost lives in the battle. Once dead, they are no longer enemies. What remains is the emptiness that prevails on both sides. So this title implies a kind of soundless world after a fierce battle. ‘眠れ’ is quiet mourning for the victims upon whose lives stands the thriving country of Japan we see today. In this sense, we even feel a sense of guilt to be rich, having sacrificed those lives.
I hope you can by now visualize the sort of nuance this Japanese title implies. I would suggest not to translate the Japanese word-to-word, but instead to come up with a completely different English title that conveys the same image you think the original Japanese tries to convey.

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Note added at 2日16時間 (2016-03-08 20:44:58 GMT)
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I gave it a thought and came up with an idea that a title like ‘A Requiem over the Blue Ocean’ may work in this case.