Nurikabe

Also called Cell Structure and Islands in the Stream, Nurikabe is named after an invisible wall in Japanese folklore. The puzzle is played on a grid of cells, some of which contain numbers. Cells are initially of unknown color, but can only be black or white. Two same-color cells are considered "connected" if they are adjacent vertically or horizontally, but not diagonally. Connected white cells form "islands", while connected black cells form the "sea". The challenge is to paint each cell black or white, subject to the following rules:

1. Each numbered cell is an island cell, the number in it is the number of cells in that island.
2. Each island must contain exactly one numbered cell.
3. There must be only one sea, which is not allowed to contain "pools", i.e. 2x2 areas of black cells.