This is not an example of new nature writing. At best, it's a travel book. At worst, it's a documentary of how he overcame his crisis of self.
It's not about nature at all. It's about one man's experience with sailing around the coast of Britain, trying to deal with his disillusionment with his home country, the fall out of a divorce, and his general lack of ambition and drive in life (which he claims to have coasted through).
Jonathan Raban writes not to share a story about nature through the first person narrative, but to exorcise some of his issues. It's a sort of self-help retelling of a diary about a man going through a hard time. This is made clear by the first and last chapters of the book. The first sets out his issues. He's unhappy with Britain and the Brits, particularly the English. He's dissatisfied with his life, his upbringing, he has some daddy issues, and he's trying to work it all out. He starts out on the sea and alone. He ends up on land and with company, presumably a girlfriend, but we never get to meet her.
What the book does afford us is a different view of England, a view from the sea, and that's a treat (albeit even a rare one in this book).