create a blank canvas that matches the size of all the pages lined up and then scan the individual pages
copy/paste them onto the canvas and move them (v shortcut) so they line up, them merge them into one layer.

i think there is a special function in the file or edit menus on photoshop (i use CS5 so wouldnt know if there's been any change in CS6). you can choose an entire folder and it stitches the images together, though im not at my home CP right now so cant say what the command name is. now i dont know how good this will be with a predominantly white set of images, though if the coastlines are sufficiently dark (or as mentioned above, you use a different coloured marker on the corners) it should stitch them together, though to be honest i prefer the first method i mentioned above. it might take a bit longer but you can make sure they match up precisely