Horizontal straighten is suitable for perspective corrections on panoramic images. Let's start with this image:
Select number 1 in the curve Segments list.
As You can see, the sea horizon is slightly curved. Press the "+" button in the "Horizontal Straighten" toolbox and click at the top of sea level - see the image below:
Set control points on the curve to cover sea curvature on the horizon:
Press the button "Straighten". The result should be like this - this is a cropped image because the horizon level is not flat (see left and right upper corners):
Choose Undo from the Edit menu or press "Command Z" to return to the previous state. Select number 2 in the curve Segments list:
Now click on the Wheel button. The curve is recalculated and now has 2 segments:
Now you can precisely move control and anchor points to accomplish horizon curvature. After this setting, the image should look like: