A puzzle type.
You can find lots of nurikabe puzzles here, as well as a good explanation of the rules here and common strategies here.
There’s also a very nice automated solver here that provides justifications for its steps as well.
Note: as of writing, there is a bug in this solver that prevents you from making the grid smaller. I’ve submitted a pull request to fix this, but before that’s merged you can just manually copy-paste my change.
Below, I’ll run through a sample puzzle and describe the strategies used to solve it.
Separating islands
Here, all shaded cells are necessary in order to separate different islands from one another.
Forced expansion
There’s only one way for the 2 to expand:
Reachability
The two newly shaded cells are not reachable:
Additionally, the new dotted cell must be unshaded, otherwise a whirlpool (2x2) will be formed.
After filling barriers around the 2 and another application of reachability:
Trial
Here, we have a choice: the top-right dotted square can be reached either from the 5 or from the 6. However, if we use the 5, whirlpools are necessarily formed on the left side like so:
Therefore, we must use the 6:
Connectivity
The newly shaded cell must be filled in, otherwise the shaded cells cannot form a connected component. In particular, there cannot exist any 8-connected cycle of unshaded cells belonging to different islands (or edge-to-edge paths of the same fashion).
From here, the solution is reasonably guessed:
There’s no shame in guessing Nurikabe puzzles; even the automated solver is unable to solving some of the 7x7 puzzles without guessing!