The poem Seafarer is basically a two part poem. I the first part the narrator is describing his excitement, fear, hardships of being out at sea. He compares his wanderlust of the sea life to life in the city. In the second part of the poem the narrator begins to talk of God and what is involved, or how a soul must act to receive the grace and the blessings of God.
There is a very precise and in-depth summary of this poem at the link below.