In the two poems, the journey is a metaphor for life and passage into afterlife. In Ulysses Tennyson talks about how his life is nearing its end and how he's old now while in crossing the bar he talks about dying and going into afterlife. Both poems use the motif of journey on open seas to describe venturing into the unknown, that is, how life passes quickly as a journey does and the next stop on the journey is afterlife.
