In the early 80s, I read almost every book in a juvenile fiction series of adventure stories. Covers we’re likely library hardbacks so probably not helpful but were green with line drawings of multiple kinds of adventures. Books includes some line illustrations. I think the series may have had multiple authors, possibly different kinds of adventures, but I most remember the diving ones. Young adults learning how to dive, learning about the bends, exploring, mostly ocean research, I think, but maybe combating poachers or sea criminals?!? I think of a combination of “Flipper” and the Frank Crisp books. Series seems like it was probably from 60s or 70s.