Dogtective William and the pirates. Elizabeth Wasserman

Alex’s mum wins a luxury cruise holiday and takes her son along. Willem plays stowaway in Alex’s backpack and later in a linen closet on board. When modern day pirates take the ship hostage William’s sleuth skills save the day. Alex and William work together to foil the pirates’ plans, which include locating and stealing a treasure on the tiny island of Trommelin.
This is the second book in the series. The third, Dogtective William and the Poachers, will be released in February 2014.