Choose the water image of the 6-square pattern, the water line being the dashed line MN at its foot.

Explanation

1. 01

   Water along MN, below the grid, swaps top and bottom but not left and right: a square in row r moves to row $6 - r$ and stays in its column.

2. 02

   So the shaded squares (1, 1), (1, 2), (1, 3), (2, 2), (2, 3), (3, 3) (row, column) move to (3, 3), (4, 2), (4, 3), (5, 1), (5, 2), (5, 3).

3. 03

   Figure 2 shows exactly these squares.