The scenery and the setting are important because they initially establish the landscape as a place of beauty, harmony, and natural innocence. They boys arrive on an island with no effective adult supervision. This is a child's paradise as long as they can work together and get along. However, despite this potential children's utopia, the boys eventually devolve into chaos. This is one of the main dichotomies: the potential to be civilized or savage. Neither Ralph's leadership nor Piggy's logic can stop this destructive decline.
We can look at this as an allegory of The Garden of Eden. It is a potential paradise as long as the boys remain civilized. Golding stresses the beauty of the island to underscore how it has this potential to be a paradise. Likewise, the entire world is a potential paradise if we treat it, and each other, with respect and gratitude. Golding may be adding a political allegory here, comparing the boy's decline to savagery with the atrocities of World War II in the supposed "civilized," adult world. So, the insistent description of the scenery is to establish this idea of a utopia, a paradise; this can only be corrupted by evil, greed, etc.
The rise and fall of the ocean represents two things: the promise of rescue and the impossibility of rescue. This dichotomy parallels other dichotomies in the novel: i.e., the island as paradise and as savage land. If a rescue is to come, it will be from the direction of the ocean. But the ocean is also the barrier between the boys and civilization. These dichotomies, balances of opposites, are crucial themes in the novel:
This was the divider, the barrier. On the other side of the island, swathed at midday with mirage, defended by the shield of the quiet lagoon, one might dream of rescue; but here, faced by the brute obtuseness of the ocean, the miles of division, one was clamped down, one was helpless, one was condemned . . . (Chapter 7)