The sea in this poem is a living, hungry creature, and it lures you in. It starts out as a "vast plain of liquid death," yet the water is also glistening and mesmerizing. That mix of danger and beauty is the heart of the poem, which states plainly that the sea is "as deadly as it is beautiful."
The poem speaks to "you," so the reader becomes the person standing at the water's edge. The feelings shift from confusion ("perplexity") to fascination to a strange calm. Even when the ocean growls, "You don't seem to mind." By the time the water seems to grimace and laugh, the pull is too strong, and "You jump in." The last line, "The ocean claimed it's newest victim," turns that choice into a kind of trap, as though the sea was always going to win.
The mood is eerie and fatalistic. It feels like a dark fairy tale about temptation, where something lovely hides a threat and the warning signs only make it more alluring.