The speaker is a pirate, or at least talks like one, and the first line sets out what that life looks like: attacking, raiding, plundering. Those three verbs come quickly and sound confident, even proud. But the second line turns on the word "But," and the mood shifts from bold action to a quieter moment of doubt.
What the speaker wonders is whether anyone else out there is like them, anyone who knows what it feels like to be "so free." That freedom is the heart of the poem. It seems to be something the speaker treasures, yet it also comes with loneliness. Having no limits on the open sea may mean having nobody who really understands you.
The short lines and simple rhymes (plunder/wonder, me/free) give the poem a light, almost sing-song feel, like a sea shanty. Underneath that playful sound, the mood is thoughtful and a little wistful. The title suggests this is one part of a longer sequence, so the speaker's voyage likely continues beyond these four lines.