A row or column that already has all its water is dry everywhere else. A row or column with just enough open squares left is water everywhere else. A line numbered 0, or numbered with its full length, is the simplest case.
This is one of the Aquarium techniques. Each page shows its technique on the same 10×10 from the game, Hard level 16.
The rule
- Count the water squares in a line, and the open ones.
- If the water already matches its number, every open square is dry.
- If the water plus the open squares exactly matches its number, every open square holds water.
In a real puzzle
On the 10×10, it’s the very first move. Row 1 is numbered 0, so every square in it is dry.
How often you’ll need it
On every one of the game’s 150 levels, and more than any other technique: 8.9 times per Easy level on average, 12.9 on Medium and 16.7 on Hard. It’s 18 of the 43 moves on the 10×10 here.