Real talk, Shmoopers. We don't know much about this speaker. Could be a dude. Could be a lady. Could be Frost himself. We get no details, no scoop, no personal info.
So what we'll focus on here, then, is the speaker's third person narration. We like to think of this speaker as relatively detached—the kind of narrative voice that allows us to see the seascape and the people without any sort of personal bias. He kind of sounds like an impersonal narrator who's painting the poem's setting for us while revealing some key ideas related to truth seeking. And that's about it.
Even if he sounds a bit disapproving of the people staring at the sea all day, he does so in a way that's not too emotional. That final rhetorical question, "But when was that ever a bar to any watch they keep?", even sounds a bit accepting of all the staring at the sea, which adds to the speaker's unbiased and unemotional tone.
Why So Detached?
Imagine if we had a more emotional and personalized speaker, or maybe even the perspective of one of the people along the sand. It would be more difficult for us to see the folly of the people watching the sea, since it's kind of hard to notice our own foolishness. It's easier to spot flaws in others, right? So by having a more unbiased speaker, we're given a fuller perspective of what's really going on. Plus it's easier to imagine ourselves as the people along the sand, which extends the poem's meaning to all of us rather than just a limited perspective.
He's also got some colloquial diction, which reminds us we're talking about modern times in a casual way. He says things like "turn their back" and "wherever the truth may be" which sound quite low-key, without the frills of super ornate language.
In that sense, our speaker sounds cool and collected without too much figurative language or fancy fluff, which might make the poem sound a bit too high falutin'. So just like the balanced form and meter that we see, the speaker also sounds well balanced, too.