From The Rime of the Ancient Mariner:
Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink.
What is the meaning of this phrase in plain English? What are "boards" here? Sides of the ship? Both sides of the ship were trying to get away from water? I can't understand this.
You are correct in that the boards are the planks of wood from which the ship is constructed. Although nautically, "boards" is more often used to refer to the decking than to the hull.
In hot dry conditions, wood will shrink. It was common in the 18th Century for wooden sailing ships travelling from Europe to the tropics to suffer shrinkage of the deck boards. This often required the decks to be recaulked to prevent leaks.
Later, wood treatment improved. Significantly reducing the risk of such shrinkage.
So the phrase is being used to set the scene of hot weather.
If you are willing to believe that the poem is an allegory of Coleridge's seven days of creation / epochs of the Earth (as described in notebook entries circa 1818/1819 and a more extensive entry dated 27 May 1828 anticipating a Magnum Opus of his own), especially in light of Coleridge appending a long quote from Thomas Burnet as an epigram to the 1817 Sibylline Leaves publication of Rime, then the line can be read as a metaphor. Thomas Burnet, in his "Sacred Theory of the Earth" (likely read by Coleridge in 1796 and mentioned in an ambitious yet rare early notebook entry in the same year), compares planet Earth to a ship in the sea of space. Moreover, the generally shared view between Isaac Newton and Burnet as evinced by a letter from the former to the latter, was that the landscape formed through "ye breaking out of vapours from below before the earth was well hardened, the setting and shrinking of ye whole globe after ye upper regions or surface began to be hard". That is to say, the water was everywhere, but in vapour form and thus impotable, and that planet earth did shrink. Burnet then refers to the sea as a kind of "slime" during this evolutionary process. See Briggs/Toor in Journal of Romanticism, Vol 2
All the boards did shrink - the fresh water barrels were all empty, and the barrel staves were shrinking as they dried. Though humid at sea, the barrels were losing their moisture. Being empty, there was no potable water.
