out to sea, takes place at high tide, when the tide "moving seems asleep" because it is "Too full for sound and foam." In other words, at high tide the water will be smoother and calmer, as the poet hopes his death will be also. The tide here is a fitting symbol for the transition from life to death, because the tide is controlled by the gravitational pull of the moon, thus implying that the speaker's death is inevitable and also that the speaker feels pulled towards his death.

In the third stanza, the speaker references the "dark" which will follow sunset. The dark here is a symbol of death and implies that the speaker expects death to be perhaps something like a void.

In the final stanza, there are references to water to symbolize the inevitable flow of time. The speaker refers to "Time and Place" as a "bourne," meaning a small stream. He then says that this stream will flow into a "flood" which "may bear (him) far." The flood that the stream flows into here symbolizes the all-consuming nature of death. For every man, the poem implies, the stream of time will eventually flow into and become an all-consuming flood, which in turn will pull each man down to his death.

Allegory is defined as any work of literature that, upon interpretation, reveals a deeper—and often moral or political—meaning. In order to answer your question, it’s important to understand that some scholars also use the term "allegory" when referring to a complex metaphor.

Alfred, Lord Tennyson’s famous poem is one such metaphor. In the text, the speaker compares sailing out into an unknown sea with the journey into the afterlife.

The speaker embarks on his trip at sunset, which is used to indicate it is the end of the speaker’s life—in other words, he has reached old age. He hopes that no one will be sad over his gradual drift toward death, which he describes as “darkness.” The bar represents the luminal space between the earthly realm and a spiritual one, and once the speaker transverses this boundary, he will have left life behind him.

The pilot that the speaker mentions at the end of the poem could suggest a guide who leads the speaker to his final destination, or it could be an angel or other spiritual being.

"Crossing the Bar" focuses on the necessity of confronting life's ups and downs with stoicism and manly honor.

This manly honor was perhaps the central component of gentlemanly virtue in the Victorian and Edwardian periods. The British believed the hallmark of a true gentleman was his ability to regard life almost as a game, and to accept its ups and downs in the spirit of "fair play." Indeed, a lot of scholars, like Paul Fussell (The Great War and Modern Memory), have analyzed how this deeply ingrained collective understanding of honor affected the behavior of British soldiers in World War I.

While Rudyard Kipling's "If" (written about five years after "Crossing the Bar") is probably the most widely known poetic examination of this type of honor, the theme was also important to Tennyson, who makes it a theme in his famous "Charge of the Light Brigade." It is also an important theme in his cycle of Arthurian poems, "The Idylls of the King."

Tennyson's central image in the poem is that of a ship setting out into the ocean. To reach the ocean, it will have to cross a sandbar. Crossing the sandbar is a metaphor for death, because death, like the ocean, is a great unknown. 

In the first stanza, the narrator realizes that night—which represents the end of his life—is approaching. Instead of complaining that night has come too soon, or complaining about the unfairness of it all, he simply accepts the evening star as "one clear call for me." Furthermore, he hopes that when his ship sets out toward its final destination, it does not do so when one can hear the "moaning of the bar." 

As we see in the second stanza, the narrator hopes for a tide "too full for sound or foam" to take him toward the great crossing. (If you are a fantasy literature fan, you might notice some parallels with the way Tolkien talks about death in The Lord of the Rings.) Once again, the narrator is demonstrating his cheerful and willing acceptance of death.

This acceptance is reiterated in the third stanza. Once again, the narrator assures us he is ready to heed the call of the "evening bell." He also shows that he is a proper Victorian gentleman because he urges his loved ones to have "no sadness of farewell/When I embark." Tennyson's intended audience would have understood that this stoic acceptance was a sign of the narrator's rationality, education, and moral virtue. 

In the famous final stanza, the narrator expresses the hope that he will "see my Pilot face to face" after crossing the bar. The narrator doubtlessly hopes that the Pilot will be impressed with his sporting attitude and unwillingness to be afraid of death, even though "the flood may bear me far."

The theme of Tennyson's poem centers on the idea of maturation and growth.  The journey that Tennyson articulates in the pom is one where courage and maturation are needed in order to effectively cope with the reality that lies outside the harbor, into the sea.  The "bar" that has to be crossed lies in this voyage, and being able to understand that one might not be able to fully control it, but rather be able to accept it as a part of being in the world is where the poem's strength lies.  The unknown, and what lies outside the realm of the harbor, cannot be tamed nor can it be avoided as all journeys enter "the boundless deep."  This cannot be averted and is a part of the ship's purpose, and the journey of consciousness.  Tennyson suggests that this is a phase that must be passed, with faith placed in the "Divine and Unseen Who is always guiding us.”  In this, the poem's thematic application to death and dying becomes something that resonates throughout, making the poem so significant.

The speaker provides a theme of death as a journey. He likens the venture as traveling on the sea, hoping to eventually meet the Pilot of his life when he crosses over "the bar." There is also a theme about living life fully, taking advantage of every opportunity so that there will be no "sadness of farewell" when his life is over.

The overall metaphor compares death to a journey into the sea.

The diction of the poem is fairly simple. There are no words of more than two syllables in the poem, contributing to the idea that the process of dying is universal, ultimately understood by everyone. The words also rely on a great deal of alliteration and assonance, which echo the mournful sounds of a boat on the sea. Consider the whispers in this line:

But such a tide as moving seems asleep

Then the hollow echoes of this line:

Too full for sound and foam

These are some powerful symbols in the poem:

• The Pilot: symbolizes God, the one who has been guiding his ship all along but who is unseen until death.
• Sunset: symbolizes the dying process; sunset is a beautiful transition that ends the light of daytime just before the birth of night.
• Home: symbolizes eternity, which is where his ship is headed—and where he originated from.
• The bar: symbolizes the transition between life and death, just as a sandbar is a barrier between land and the sea.