The theme of "The Tide Rises, the Tide Falls" by Henry Wadsworth Longfellow, as most often interpreted, is that nature is indifferent to the life of humans and when a human dies, nature still continues its cyclical pattern. In this case, the loss or potential death of the traveler has no effect on the natural pattern of the tide.
Henry Wadsworth Longfellow wrote "The Tide Rises The Tide Falls" near the end of his life and long career. It is possible that he was reflecting on his old age, and how life and nature will continue on without him. When the traveller in the poem never returns, the tides still go in and out as if the traveller was never there to begin with. Longfellow is most likely hinting at the cyclical pattern of nature, which is indifferent to the life or death of a single person.