The meter of "Crossing the Bar" is all over the place; in fact, there are three different kinds of meter. But let's start small, shall we?
Check out line 12: "When I embark." Hear that daDUM daDUM? That's iambic dimeter, which is poetry's fancy pants way of saying the line contains two iambs—no more, no less.
Then there's line 2: "And one clear call for me
That's followed by line 3, which goes, "And may there be no moaning of the bar." We have five daDUMs here, which means Tennyson has given us a line of everyone's favorite meter, iambic pentameter.
So why can't Tennyson just pick one meter and stick to it? That's a fair question. We'd call him flighty and sloppy if we weren't sure he was up to something. Tennyson is forever making mischief. In this case, he's using those varying meters to mimic the back and forth flow of the ocean—one minute it's a short little wave, the next, it's more of a swell. And check out the way every other line is indented. That does the same thing, too, huh?
The Big Blue Picture
Now that we've given you the nitty gritty, let's zoom out. "Crossing the Bar" is composed of four rhyming stanzas of four lines each, a.k.a. quatrains. Each of those quatrains has alternating rhymes, giving the entire poem a rhyme scheme of ABAB CDCD EFEF (Lady) GAGA.
The rhyme scheme of the poem complements its themes in one really cool way. Think about it like this: The poem suggests that death isn't necessarily the end, but rather that there is some other life beyond death, right? In the same way that life goes on after death, the poem's lines continue even after they end, in the form of another line that rhymes with it. Lines are married together, just like life and death are. So even though the line ends, it's never really over.
And did you notice that the A rhyme returns in the last stanza? The poem comes full circle, in that words in the last stanza rhyme with words in the first stanza. So much for endings, right? In "Crossing the Bar," death—and poetic form—are all about new beginnings.