First steps

Ones, separators and unreachable cells.

Ones are complete islands: shade every neighbor.

A cell touching two different numbers (or two islands) must be sea, because joining them would make one island with two numbers.

For each island, count how far it can still grow. A cell too far from every number can’t be land, so shade it.

Keep the sea connected: if a shaded group has only one way out, that exit must be sea as well.

Practise it

The Hint button on every puzzle explains the next step in the same plain language, so you can learn as you play. Play a Nurikabe puzzle.