The original photo of Hilo Bay, Hawaii is shown here. The image looks tilted, but what should be level: the horizon line? The far shore?

A careful look reveals a sea wall, which we can assume has a level top edge.

 

Zooming in with the magnifier makes it easier to select the top edge with the marquee tool.

 

Selecting File -> Automate -> Upright reveals that the edge is tilted 1.9 degrees. Clicking OK rotates the image to make the top of the sea wall level. We then cropped the image to remove the rotated image edges.

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