Juliet writes to Sidney and Piers and tells them that she's been spending all her time researching Guernsey. She discovered one travel book on the island. The author is very unserious and wrote poems about the flowers, the cows, the "simple folk of the country parishes," and the sea. The co-author seems to hate Guernsey and have less of an inclination towards poetry. She wrote about the island's history and was incensed that the Channel Islands technically belong England, but don't pay taxes and don't have to listen to England's government.
In the case of Guernsey itself, the sea means that it's able to exist independently from either England or France, which it's geographically closer to. This in turn offers the people of Guernsey more freedoms to dictate their own lives. In the case of the war, this meant that Guernseymen weren't conscripted like other men in England; they joined the war effort voluntarily or not at all.