Magic Tree House #9: Dolphins at Daybreak by Mary Pope Osborne (Good, 1997, Pbk, 67 pgs, Scholastic)

In good condition: cover and pages have some wear; pages are lightly age-toned. ISBN 059070635780871824x

It's sink or swim for Jack and Annie when the Magic Tree House whisks them off to the middle of the ocean. Luckily, they find a mini-submarine on a coral reef. Unluckily, they are about to meet a giant octopus and one very hungry shark. Will the dolphins save the day? Or are Jack and Annie doomed to be dinner?