Part II Summary

Get out the microscope, because we’re going through this poem line-by-line.

At least time doesn't stop after he kills the albatross. The sun keeps rising and setting just as before, and the weather remains misty. Since the sailors are now traveling north instead of south, the sun rises on the right and sets on the left, instead of the other way around, as in Part I, Stanza 6.
But, leave not doubt, that bird is as dead as a doornail. The sailors' favorite pet is gone. If you have ever read any other literature about sailors, like Melville's Moby-Dick, you might know that they take their good luck charms very seriously.
The sailors are convinced that the bird brought them the good winds, and they all agree ("averred") that the Mariner has done a bad, bad thing.
But then the mist goes away, and the sailors change their minds. Instead of bringing the good winds (hooray!), the sailors decide that the bird was responsible for the fog that was making it so hard to see (boo!). They now blame the bird for bad luck. Those fickle sailors.
Everything is going along quite well for the crew. They carve the mounds or "furrows" of the waves with the wind at their back. They make their way into uncharted territory.

One tip for reading this poem: conditions change really fast. It only took a stanza for the sailors to decide that the albatross was really a bad luck charm instead of a good one. Here, it only takes a stanza for the weather to turn from delightful to dreadful.
In short, they lose the good breeze at their backs, and without a breeze to fill the sails, the ship can't move. Suddenly, the "silence" of the uncharted waters sounds very ominous.
The sun is small and "blood-red": it looks very far away. The sky has a strange fiery color, but their main problem is a lack of water. If they don't find some kind of land (or, heck, ice), they will all die of thirst.
There's no wind. Literally. Not even a tiny gust. The ocean looks like glass, and the scene is so motionless that it could be a painting.
Without any water, even the "boards" – the wood planks of the ship – start to dry up and "shrink." So…thirsty!
Um, so, sailors, what was that you were saying about being glad that the albatross was dead?

When the world gets dry, the ocean starts to "rot" from the dryness. Think of a pond that is drying up, and how it turns brackish (extra-salty) and starts growing nasty algae. The ocean around the ship is undergoing a similar transformation. Its surface turns "slimy" and gross, slimy creatures start to appear.
These creatures aren't fish: they have "legs." Are they walking on the water, or what? Hard to tell what's going on here, but the poem is beginning to turn strange.
Crazy, disturbing lights start to appear at night, and the water "burns" green, blue, and white. If you wanted to be scientific about it, you might guess that the Mariner is seeing the phenomenon of "phosphorescence." Some kinds of algae and tiny animals can literally "glow" in the water in certain times of year.
But Coleridge isn't being scientific, he's being supernatural. Some of the sailors start to dream that a spirit deep under the ocean has been following the ship ever since they left the Antarctic. Needless to say, it's not a happy, fuzzy spirit.
The crew becomes so thirsty that they stop producing saliva and cannot talk. But they can still give the stink-eye to the Mariner. "This is all your fault."
In one of the poem's most famous images, they hang the dead albatross around his neck.
Side note: First, when did they pick up the albatross? We never heard about that one. Second, that albatross must really stink to high heavens.