Normal sudoku rules apply. Shade some cells in the grid. Those cells are part of the "coral". The rest of the cells are "water". All coral cells are orthogonally connected. All bodies of water connect to the edge of the grid. No 2x2 is entirely water or entirely coral. Digits do not repeat within cages, and the sum of the coral cells within each cage must equal the sum of the water cells within that cage. The digit in a circle gives the number of cells within its cage which match its colour (coral or water). The total includes itself.