A speaker is lost in someone's blue eyes and tries to say how deep that feeling goes. The comparison is simple: the eyes are bluer than the ocean on its clearest day, so vast and so vivid that nothing in nature quite matches them. Then the image shifts. The speaker doesn't just admire the sea, they picture themselves as a boat moving through it, and not just any boat but the only one.
That last wish is the heart of the poem. "I'd kill to be" is an exaggeration, but it shows how possessive and intense the longing is. The speaker wants to be the one person who gets to travel through this person's gaze, close and exclusive. There's tenderness in the idea of sailing, which feels calm and free, yet the jealousy underneath gives it an edge.
The mood is dreamy and romantic, with a hint of obsession. It's a crush, or a deep love, expressed in a single breath. The title, "blue eyes," keeps the focus on that one detail the speaker can't stop looking at.