The Whisper (The Riverman Trilogy)

Twelve-year-old Alistair Cleary washes ashore in Aquavania, a magical realm where children use imagination to make whole worlds. The creators have vanished, leaving those worlds to collapse. Alistair wants to find his friend Fiona Loomis and return home, but must move from world to world while facing starlight animals, a boy king, and astronauts who trade in riddles. His search reveals unsettling truths about himself, others, and reality.