The treasure of Pirate Frank by Mal Peet and Elspeth Graham
A boy and his dog set sail in search of treasure, braving stormy seas . . . mountains snowy and cold . . . the forest where the monkeys swing . . . the swamp where the bullfrogs sing . . . and other challenging obstacles, before finally reaching the end of their quest.
But wait! Pirate Frank is A GIRL! And she doesn't want to share. Back home they go, retracing their steps before sailing away to dream of the treasure they've lost.