This is the last of the original Goosebumps books that hasn't been released with new cover art (see my previous works for the others), still plenty of Series 2000 books to work. This is Book 58: Deep Trouble 2.
It could be argued that this book was retconned by the Horrorland book, Creep From the Deep, but not what we're here for. Billy and Sheena Deep are vacationing at sea with their Marine Biologist Uncle, Dr. Deep, Dr. D for short. They soon discover marine life significantly larger than their recorded sizes and soon they encounter the mad scientist responsible, Dr. Ritter. Ritter will stop at nothing to keep his secret...
©RL Stine/Scholastic