Just bumping this in case anyone has any new ideas or suggestions. This interest is going as strong as ever and DS is always hungry for more. We have pretty much tried out everything documentary and ocean-related on the Netflix channel that looked plausible (verdict: nothing beats Blue Planet). DS is reading on about a 3rd or 4th-grade level now. He went nuts for the Magic Schoolbus and Magic Treehouse ocean-themed books, but has read them all.