These two poems, though seemingly very similar at their beginnings, in fact have two very different messages and attitudes regarding faith, in addition to differing in their tones and sentiments about life, trial, hardship and despair.
Toplady's poem begins with a description of sailing out to sea. During his poem, he speaks to God, and blesses God for His great bounty and mercy, and declares that even trials drive "us nearer to home," and that in the midst of those trials, God" "tender mercy" will "illumine the midnight of the soul." In other words, when hardships occur, they only serve to bring us closer to God (or home), and that through them, God does not abandon us, but gives us further light to guide us in the darkness ("illume the midnight of the soul"). He continues his declaration of faith by saying that even death does not drive him from God; rather, that "faith alone" makes life worth living. To summarize, Toplady's poem is filled with calm and joyous expressions of faith in God; that He is there through our trials, and that to have faith is what makes life fulfilled.
Compare that to Arnold's poem. He too starts off calmly and reflectively looking at the sea and pondering it and life, just as Toplady does. So, they both discuss the ocean and the scenery with respect and descriptions of beauty. However, of faith, Arnold believes that the entire world is devoid of faith or hope; he says that he can "hear its melancholy, long, withdrawing roar," and in its absence, mankind is left bereft and alone, with only each other for comfort. He declares there is no beauty, "nor joy, nor love, nor light, nor certitude, nor peace" left in the world, and that we are alone, without God or faith, on the "darkling plain" as armies fight each other to the death. It is a much more pessimistic viewpoint of the world, and describes a world without God or faith left in it. On the other hand, Toplady's world described is filed with faith and God, and in his mind as a result, joy and love.
I hope that those thoughts can help to get you started; good luck!