Around 1967(?) I got a new hardback from the library which was profoundly eerie, otherworldly and way more serious, anti-social and disturbing than most young adult novels of that time. It might have had only two human characters. A boy has to go live with his depressing silent uncle on the coast of New England. To escape his loneliness, he goes swimming with the seals at night, coating his skin with oil to keep him warm. He gradually merges with them and if I recall correctly, he doesn't come back. This book was downright psychedelic, moody, and unique. I've never seen it anywhere since.
I asked this question somewhere else, years ago, and it hasn't been identified yet: http://loganberrybooks.com/stump-s.html (S-308)
I'd love to find the book but no idea what the title is or who wrote it.