Where It All Goes Down
It's a beach day for sure in "Neither Out Far Nor In Deep." But we're not seeing any beach umbrellas and bathing suits. The poem's setting is more like an austere beach with folks contemplating life's mysteries while a solitary gull stands by their side. At one point they spot a ship out at sea though, so it's not like we're on a deserted island. This is a beach you might see, well, anywhere.
Even though it may seem like we're at the shore, we get the feeling that this is more of a symbolic beach with bigger meanings and ideas going on. The sea isn't just the sea in this case. Instead, it represents the infinite world that we can't really see or understand. Likewise the land is symbolic of all the stuff we can see and do see on a regular basis, although it may "vary more."
But the speaker is also careful to paint this particular setting in a way that's deliberately more abstract and ambiguous, in order to point out some of the habits that affect all mankind. So the generalizing descriptions like, "the people along the sand," allow us to imagine ourselves—and even the speaker, perhaps—as a part of that sea-gazing group. And since we all catch ourselves now and then contemplating some of life's bigger questions, it's not hard to imagine ourselves on that beach "turning our backs to the land."