Starting at the beginning of the poem, the seafarer is melancholy to the point of being profoundly depressed. He is depressed because he is stuck in a life on the ocean. He even refers to it as a kind of prison. He adds that the man who lives on land can not know the plight of the seafarer's life.
No man sheltered
On the quiet fairness of earth can feel
How wretched I was, drifting through winter (12-14)
The seafarer feels compelled to this life of wandering by something in himself ("my soul called me eagerly out"). He wonders what will become of him ("what Fate has willed"). Around line 44, the reader can take the seafarer's words as a Christian allegory. That is to say that life is a journey and each individual must deal with wandering, deprivation of comfort, and in the end, accept what fate/God has in store for him/her. By accepting his fate as a seafarer, he is acknowledging his role in God's divine plan.
Therefore, he looks at his sea life as something of a crusade he must undertake on God's behalf. Thinking on this, his spirit for adventure is renewed but, again, he invokes what a tortuous life it is. So, his mood, generally speaking is one marked by sever melancholy with short bursts of optimism based upon his faith in God and the "hope in Heaven."