Tennyson's "Crossing the Bar" begins at "Sunset," which is a time of day often used in poetry to symbolize dying or death. Indeed, the sun setting over the horizon, and thus the light of day fading into the dark of night, seems a fitting symbol for the end of a human life.
The "bar" mentioned in the poem's title and in the third line of the first stanza refers to a sandbar, and it symbolizes the transition between life and death. The speaker says that he hopes there will be "no moaning of the bar / When (he) put(s) out to sea." The implication here is that he hopes his own transition from life to death will be smooth and without the pain or discomfort implied by the "moaning."
In the second stanza, the speaker hopes that his journey towards death, represented in the poem as a journey 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.