Idea for a book
Following catastrophic sea level rise, all that's left of a small seaside city is the cathedral. The entire, diminished population now live in it, with a shanty town hanging among the buttresses and arches and splayed out across the great roofs. The townspeople live off the sea and by trading the treasures they dive down to discover in the depths of the cathedral with merchants who sail out to the stone island. One boy is particularly good at this treasure hunting and a group of pirates visiting the cathedral kidnap him and take him back to the flooded big city, where they want him to dive for treasure in a submerged bank. He must survive the chaotic city, outwit his captors and get back to safety on the cathedral.