Nephi is prophesying about the three days of darkness which covers the land of promise after Christ is crucified. The term, "isles of the sea," is a term which Nephi uses to refer to the land of promise. We think of the Americas as two great continents. However, it is completely natural that Nephi, having sailed to the promised land, should refer to it as an isle of the sea, we have been led to a better land, for the Lord has made the sea our path, and we are upon an isle of the sea. But great are the promises of the Lord unto them who are upon the isles of the sea; wherefore as it says isles, there must needs be more than this (2 Nephi 10:20-1). Every time Nephi refers in a prophesy to the "isles of the sea," he is thinking about the land of promise.