This one's easily done with the cloning (stamp) tool. Zoom in close on the scratch and use a brush size that's just a little larger than the scratch is wide. You then ALT+click in the image area next to the scratch and the paint over the scratch with the stamp tool. Use the aligned mode for the cloning stamp and ALT+click a new source spot when the cloning isn't lining up well with details like angles of the wave shadows.
Don't use too large a brush. Create a new one if necessary. Get comfortable doing the ALT+click to pick the best source for cloning. When you're using the aligned mode, the distance and angle from your source location to your tool center remains the same until you pick a new source. Change the source and angle as often as necessary and clone from either side of the scratch as needed.
It took me about 20 seconds to clean the scratch from this low res image. On your high res scan it will take a couple of minutes to do it well.
I usually use a different method of dealing with scratches. I copy the original layer. I then go to the background layer and apply Filter>Noise>Remove Dust and Scratches. This gives a soft look to the bakground, which you can then get rid of by merging the copy with the background with the blending mode changed to Darken. When the sharp upper picture merges with the soft background, only the pixels that have been darkened by the filter will show through. Martin Evening shows a similar technique in "Adobe Photoshop 6.0 for Photograpers" on p. 226, using the history brush rather than duplicating layers.
This is a fast technique and may be helpful if you have a lot of dust and/or scratches to remove from an image, but since the remove dust & scratches filter works by blurring, the technique will cause some softening of the image even though the blur is only applied to pixels that can be darkened. The wider the scratch you want to eliminate, the greater the blur you'll need to use. Dark dirt and scratches won't be fixed by this technique.
I reserve this kind of treatment for just a few dirty images. I find it's worth the time for most images to hand-spot a few dust flecks or scratches rather than soften the whole scan with broad techniques. Photoshop can't tell a dust fleck from a white water droplet or highlight, so some images can have significant detail lost via dust and noise elimination tricks.
In addition to "roll-your-own" techniques, there are plugin filters that can automate retouching to some extent. You can also turn a technique you use often into an action script.
The best automated cleaning software like Digital ICE uses information from an infrared channel that is built into some scanners (mostly Nikon). The IR channel lets the software fix real blemishes without taking out image detail that falls within the definition of a dust or scratch to more mindless correction methods.
If you're worried about other spots in the image, make a selection around the scratch before applying the filter.
Dark spots can be removed using the lighten blending mode.
Of course we should use the technique that's best suited to the image we're dealing with. One of the nice things about Photoshop is that it's easy enough to try several techniques and then pick the one that does the best job.
Generalized scratch removal would probably have removed the fishing pole ends that are next to the guy with the hat. The white foam flecks and small highlights on the stairs would also be at risk. Someone just starting with Photoshop should get comfortable with the basic tools like the clone stamp and then it will be easier tackle the more advanced techniques described above.