The most obvious comparison between Alfred Tennyson's "Crossing the Bar" and Robert Frost's "Once by the Pacific" is that both of them deal with water, specifically the sea.
Of course, Frost's sea is much more turbulent and even violent than Tennyson's. Frost's ocean crashes into a cliff, and
[t]he shattered water made a misty din.
Waves crash upon waves, and the speaker believes the land is lucky to have the double protection of cliff and continent to hold back the foreboding weather that seems to be approaching. He says:
It looked as if a night of dark intent
Was coming, and not only a night, an age.
The language of this poem represents the impending violence of a fierce ocean which is churned up by a powerfully dark and portentous sky.
This poem is fourteen lines long, so of course we think of a sonnet. When we examine the form, however, we understand that the rhyme scheme fits neither the Shakespearean nor the Petrarchan form. Instead it is comprised of seven rhyming couplets. This rhyme pattern kind of sounds like a march, adding to the ominous aspect of an approaching storm.
Frost's poem is mostly literal: the speaker is watching a storm approaching, and it causes him to have some fear and trepidation. He mentions God at the end, suggesting that God is in control of all things and will not destroy the world with this relatively small storm,
There would be more than ocean-water broken
Before God's last Put out the light was spoken.
In contrast, while the water in Tennyson's poem is real, it is used in a much more symbolic sense. The title, "Crossing the Bar" is a metaphor for crossing from life to death, so the water in the poem is much calmer but much more significant. In fact, it is calm enough for the speaker to "put out to sea," a symbolic reference to making the metaphorical final journey from life to death.
There is little physical description of the water, only the tide which is barely moving. The only darkness in this poem is the literal darkness of the day (which of course is symbolic of a life reaching its end), in contrast to the moving and temporary darkness of the storm clouds in Frost's work.
The poem is written in four stanzas with every other line rhyming. It is a steady, even, symmetrical rhythm, evocative of the flow of a gentle tide. Even more than gentleness, this rhythm implies a naturalness to the events presented in the poem. This also fits with the theme of passing from life to death, a natural part of everyone's life cycle.
The most significant difference between the Frost and Tennyson poems, it seems to me, is the use of spiritual symbolism. In Frost's work, God is present and even credited with being able to control the storms. In Tennyson's work, God (and presumably heaven, the place where he resides) is a destination for the speaker who has put out to sea. In the final stanza, the speaker says this:
For tho' from out our bourne of Time and Place
The flood may bear me far,
I hope to see my Pilot face to face
When I have crost the bar.
This is an expression of hopefulness that the speaker will find heaven at the end of his journey.
By referring to God as his Pilot, Tennyson changes the focus of the poem from the water to the journey and the destination. While Frost also references God in the final lines of his work, the approaching storm is still an approaching storm; it may be an indication of worse things to come one day, but for now it is simply a storm.
Both poems are simple and masterful.