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.
Stump the Bookseller
New format, same book stumper solving.
A long shot — the characters in the Rick Brant series live on an island, and some of the adventures are underwater-related (but only five or six of them). I don’t recall if the covers were green (half-memory says they wree red). But, for what it’s worth:
https://en.wikipedia.org/wiki/Rick_Brant
Hi
Sounds a bit like the Willard Price Adventure series featuring Hal and Rodger Hunt?
The Adventure series by Willard Price might fit your description, except it was written by one author. Hal and Roger go on scientific expeditions and collect animals for the family business, supplying zoo’s. Among the series is Underwater Adventure (1954) and Diving Adventure (1969). They did have line illustrations.
https://en.wikipedia.org/wiki/Willard_Price%27s_Adventure_series