Dark Day in the Deep Sea Magic Tree House #39 by Mary Pope Osborne illustrate by Sal Murdocca

Jack and Annie travel to the 19th century and board the HMS Challenger.  This boat is traveling the seas to discover new sea creatures in the ocean depths.  Jack and Annie fear for their lives when they are swept overboard in a storm.  The sea monster, the octopus, saves them.  Can Jack and Annie return the favor?