Removing dark spots from the sky or the sea.

One of the techniques you can use is "The History Brush Technique".

The Photoshop History Brush Tool technique allows you to clean up large areas of dirt quickly when the area to be cleaned contains very little detail e.g. the sky or other background area.

Go to Filter / Noise / Dust & Scratches... and choose a radius large enough to remove all the blemishes. The whole image will be blurred, but it's ok.

Select the History Brush and in the History Palette, click on the small square box on the left of the Dust & Scratches history. This sets the origin for the History Brush.

Now click in the previous History state, the larger rectangular box immediately above and to the right. This returns us to the unblurred image.

With the Mode set to "Lighten" paint over the dirt with the History Brush.

All traces of dirt will vanish.