The speaker keeps circling an idea of "Later," a time we put our hopes and griefs into because the present feels too hard to face. The sea is the poem's main image. Its glittering waves offer a brief moment of beauty, and perhaps that brightness is needed before the mind's noisy, restless thoughts finally go quiet. But the speaker also admits something harsher: our backs scorch, and the spirit inside us seems to lie dormant.
The mood is hushed, tired, and a little mournful. Later is described as a "dreamer" and then a "pallbearer," so it comforts us and carries our losses at once. The speaker doesn't offer a big answer. Instead, the closing lines suggest the crashing waves will have to do for now, and that we must eventually come down to earth and accept what misfortune leaves behind. The tone is resigned rather than angry, with a faint, wary hope in the shimmering water.