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!