Normal sudoku rules apply. Cells are either 'sea' or 'land'. No 2x2 block is filled with just sea or land. All sea cells are connected orthogonally. Cells with arrows are dead ends, that is, sea cells that point to the only other sea cell they touch. Land cells form 'islands' that don't touch each other, even diagonally, comprising four orthogonally adjacent, different digits, summing to 14. On an X, the two digits sum to 10, and on a red dot, they sum to 12. All arrows, X's, and red dots are shown. On an X, the two cells are both sea or both land.