I've been searching for this for years. When I was a kid (so early to mid 90s), my mom had a lot of "read-along" books for kids, which were essentially audiobooks on cassette with a physical book for you to follow along with the story. Sometimes there were words and illustrations, other times just illustrations. I don't remember which way it was for the plesiosaur one.
Anyway, I remember it having a scene and matching illustration with a young plesiosaur out in the water alone in a storm. I think he might have had a pterodactyl friend and some other friends, but I'm not sure. I don't remember anything else, unfortunately, and searching for this has almost been enough to make me believe I imagined it. Does this sound familiar to anyone else?