If the sandbar is Tennyson's metaphor for the boundary between life and death, then "Crossing the Bar" is all about crossing from life to death. But if you think you're in for a morbid poem, you're in for a surprise instead. This poem is all about accepting and embracing death, rather than fearing the dark unknown.
What Tennyson's really trying to say here is that if you relax a bit, death is as peaceful and easy as putting out to sea.
Death is not the end, but a new beginning; the reference to the sunset suggests as much. The sun sets, but it always rises again.