You asked in one of your other posts if there was a better method than using the eraser tool.
I don't know if this method is better but it is an alternative , I have found with photoshop that there is usually more than one way of achieving the end result ,it depends on what method you are happy with.
I appreciate this is overcooked but this is to show the effect and as you have image editing turned on I have had a play so I hope you dont take offence.
All done using CS5, I don't know if other versions have the same tools
Start by making a copy ( Ctrl J)
1) Straighten the horizon using the ruler tool,rt click on the eyedropper to open it up,click on one end of the horizon,draw a line to the other end,click on the end and then click straighten on the top tool bar.
2)
Use the marquee tool to select the sky down to the horizon,then layer>new layer>layer via copy.
That will put the sky on a separate editable layer.
Then layer>new>new adjustment layer ( I used levels) but at this point you can select any adjustment layer ( purple skies if you want)
The important bit here is to tick the clipping mask box in the adjustment layer dialog box,this restricts changes to the layer below the adjustment layer,if you don't do this the adjustment layer will affect the whole image.Then alter the sky to get the affect you want.
I tend to turn off the layers I am not working on so I can see what I am doing.
By working in layers you can alter various bits of the image independantly( I have boosted the foreground contrast a bit,but I like saturated colours,not everyone does.
I have taken out the blue line on the horizon but with hindsight I think it looks better with the line left in.
It took ages for me to get my head round layers and masks but they are worthwhile learning.
Regards