Symbolism, Imagery, Allegory
In this book, nature isn't just a backdrop—it's a force of, well, nature. Man is small and almost inconsequential in comparison to the natural force of the tide, the dangerous marshes, and even the calm and beautiful setting of Monk's Piece. And Arthur will not zip it about the power of nature:
I had never been quite so alone, nor felt quite so small and insignificant in a vast landscape before, and I fell into a not unpleasant brooding, philosophical frame of mind, struck by the absolute indifference of water and sky to my presence. (6.2)
He says "indifference"; we say that it almost seems like nature is out to get him. The causeway cuts him off from the town when the tide is high; the wind wakes him up in the middle of the night and makes the whole house whistle; and the boggy marsh nearly kills him when he goes to rescue Spider.
This is also true to gothic literature's tendency to go into the sublime, which is the feeling of terror and awe that you get when you encounter nature in its human-crushing form, like a super scary thunderstorm or a powerful waterfall. (Think Wuthering Heights and all those broken hearts wandering through the desolate moors.)
Arthur may have smarts, a modern job and a rational attitude, but he's no match for the brutality and beauty of nature. And he knows it.