The speaker describes himself as a captain without a ship, someone who has lost the thing that once gave him purpose. He doesn't dwell on it, though. "The past is the past" is a plain acceptance that what's done can't be changed, and the poem moves quickly toward what comes next: a new crew.
As a leader, the speaker is demanding but fair. He asks a lot of his crew, yet promises never to expect anything he wouldn't do himself. That gives him a steady, honest voice, more like a teammate than a boss. The sea works as a metaphor for hard times, and he is upfront that the journey will be rough and will take skill. But the last lines offer hope: beyond the storms lies the Sea of Tranquility. The mood is determined and quietly encouraging, a mix of regret left behind and calm confidence that effort will lead somewhere peaceful.