When I was young, I loved the book "Exploring the Deep Frontier", by Sylvia Earle; it's still available in that printing, and it looks like she has other "Deep Frontier" books available now.
There are some ocean-related early readers that DS6 enjoyed once upon a time. "Hungry, Hungry Sharks" and "Tentacles! Tales of the Giant Squid" come to mind. He's also enjoyed DK books related to the ocean. More recently, we got "The Deep: The Extraordinary Creatures of the Abyss", which is a gorgeous book.
"Blue Planet" and "Planet Earth" on DVD and Blu Ray contain some good deep-sea episodes.
