How we cite our quotes: (Line)
Quote #1
They turn their back on the land.
They look at the sea all day. (3-4)
Since the land appears to symbolize earthly life (rather than the infinite life the sea provides), we understand that the people along the sand have turned their back on the real stuff. Instead they stand and stare at the sea (mysterious stuff) all day. They're in a sort of stasis (stillness) both physically and symbolically in terms of living their lives.
Quote #2
The wetter ground like glass
Reflects a standing gull (7-8)
The wetter ground reflects the standing gull while the gull reflects the standing people. That's a whole lot of inertia (and reflection) in only two lines. So the speaker is making it clear that the people, like the gull, are at a standstill despite (or perhaps because of) all their deep thinking.
Quote #3
They cannot look out far.
They cannot look in deep. (13-14)
If all the staring led to some truth, it would make sense. But here we see the reality that they're never going to get very far in all their staring. So no matter how far they'd like to go, in terms of truth seeking, the fact of the matter is they'll still find themselves in the same place.