Crossing the Bar- Medaphor

"Twilight and evening bell,  
  And after that the dark!     
And may there be no sadness of farewell,  
  When I embark;"

Crossing the bar is like crossing in to the gates of heaven. When this man is on earth, he wonders what this journey will be like by imagining the only part of heaven that he can actually see, the horizon. This horizon separates the heavens from the earth. So then crossing this horizon, meant leaving the earth behind and entering the beyond. This also be more simple then the man dreaming about what heaven would be like. He might just be praying. He is out at sea, all alone and looking into the heavens to ask for help, crossing the horizon to talk  to the "pilot". This could explain why he was seeing the horizon go from day to night, because he is in need of rescue. This could also, explain both interpretations. He might be pleading for God to save him, or contemplating death.