The Rime of the Ancient Mariner goes through several important transformations at key points, like after the Mariner shoots the albatross, but the most important transformation is the Mariner's conversion from prideful jerk who hates large birds to pious soul who can pray for even the ugliest creatures. The albatross that hangs around his neck represents the burden of his sins, which fall away when he repents and blesses the sea snakes. However, he hasn't simply wiped away his evil deeds after this transformation. His penance continues throughout the rest of his life, every time he feels the painful urge to tell his story.
The Mariner's supposed moral is too simplistic and does not reflect the true lessons to be learned from the story. He remains blind to the meaning of his own words.
The Mariner's blessing of the sea-snakes is not a Christian conversion, but rather an affirmation of the pantheist belief that God is nothing more than the totality of nature.