After a quick read through, I would say that the general theme of the poem relates to realizing that the universe is bigger than any one of us or humanity as a whole. The sea, a constant, continues its tidal cycle with or without us. By remaining anchored to nature, we can keep perspective about the world.
Who wrote the introductory material? Even if it wasn't the poet, it's no coincidence that this person wrote that the sea "pulses" through their lives. Look for the parallel in the last stanza.
To get more into the poem, I would recommend focusing on the natural imagery and the 's' sounds. I also recommend the article "
Principles of Analyzing Sound in Poetry" to help looking as the 's' and other sounds.
I hope that's enough to get you on the path.