Getting your game ready…
Mark every cell as island or sea. Each connected island must contain exactly one fixed number and exactly that many cells. Island cells connect through shared edges. Different islands cannot touch along an edge, although diagonal contact is allowed.
All sea cells must form one connected area through shared edges. A solid two-by-two block of sea is forbidden. A numbered cell always belongs to its island and cannot be changed into sea.
Tap a free cell to cycle unknown, sea and island, or use arrows and Enter or Space. The keys 1 and 2 mark sea and island; 0 or Delete clears. Every unknown cell needs a final choice. Undo reverses a change; Restart returns to the selected board. Hint points out a rule or deduction without filling the cell.
An island numbered one is already complete, so its edge neighbours are sea. When a larger island reaches its size, the same boundary rule applies. Check that surrounding it does not cut the sea into separate pockets.
A sea region with only one way to reach the rest must keep that route. Meanwhile, an unfinished island needs enough reachable cells to grow. Compare both requirements before choosing a square that could serve either side.
Six five-by-five boards balance numbered islands and one sea around them. Island ownership and sea connections must agree across the whole board. Solved boards stay visible until Undo or another board choice. Completed puzzles and your last selection are remembered, but reopening starts the chosen board fresh.