How does his view of these water snakes change at the end of part IV in lines 272-291?

The Mariner was a bit of a jerk before his albatross conversion. He cursed the creatures of nature. He considered the sea snakes as “slimy” and unsightly. After his conversion The Mariner sees them differently,

"The great water snakes moved through the silvery moonlight, glittering; blue, green, and black, the snakes coiled and swam and became beautiful in the Mariner’s eyes.He blessed the beautiful creatures in his heart.

The man had a spiritual awakening. He loved all of God’s creatures. This transformation comes about from a series of pretty horrid sea conditions and nightmare visions. My poem doesn't really have the same line numbers as you but I hope this helps.