Osborne, Mary Pope.
New York : Random House, 
Title from eBook information screen.
"A Merlin mission."
"A Stepping Stone book."
When eight-year-old Jack and his seven-year-old sister, Annie, join a group of nineteenth-century explorers aboard the H.M.S. Challenger, they learn about the ocean, solve the mystery of its fabled sea monster, and gain compassion for their fellow creatures.
Electronic resource: http://kyunbound.lib.overdrive.com/ContentDetails.htm?ID=EA28A733-ACEA-4136-B7DA-9180B35BC24B