They kind of look like sand zombies, no? They're just standing around, staring at the sea all day, not giving a hoot about their real lives. The speaker also makes a point to generalize them by using words like "the people" and "all." So they're just one big zombie-like group of time- and life-wasters.
Line 1: Right from the beginning we're meant to see "the people along the sand" as one big group without any distinguishing characteristics. They all look the same and do the same thing, too. How boring.
Lines 2-3: They're all looking the same way and they have their back to the land. So we understand that they've got a very narrow scope in the way they see things. And they're disregarding their concrete lives on land, too.
Lines 7-8: Beside them is a "standing gull," which reflects the people wasting time standing still while disregarding their own lives. The gull, like the people, is isolated and removed from its natural purpose.
Lines 15-16: By the end we understand that all the staring and thinking about the unknowable is probably just part of human nature. Sure it may be a waste of time but it's also a part of who we are as thinkers and dreamers.