The speaker wants to understand something, but the way they try is odd: not by thinking harder, but by going under. The repeated "under and under and under" feels like slipping down through layers, and the title's "Going, going…" adds to that sense of fading, like something slowly slipping away.
Then the poem plays with a familiar phrase. "In one ear and out the other" usually means a thing was heard but not taken in. Here it sounds like understanding may not arrive through effort at all, or that it passes through without sticking. The last lines are the gentlest: the speaker drifts "with the tide / home to bed," giving in to the pull of water and sleep.
The mood is quiet, drowsy, and a little wistful. The speaker seems less frustrated than tired and accepting. Judging by the images, the poem suggests that understanding might come only when we stop pushing, or that sometimes rest is the better answer.