Please you do not have to drink more water! You should get plenty of fluids from your food and there is no eveidence that drinking more makes your skin look healthier. I hardly drink water, apart from squash, juice and coffee. I have a great lifestyle.
The dark circles are usually viens, blue and if you pull the skin you can see them. i have a pale complextion and have always had a thin vieny skin around the eyes. As I'm getting older the eye lid is more sunken and the eye looks darker and older, actually tired. There is no safe surgery for this as I would love the viens to be lazered out but location to eye isnt safe.
As for concealer I went out and bought an expensive one 'By Terry' but to be quite frank I dont find it any better than the No7 one I had. The trouble is if the skin is thin, you dont want a thick heavy filler than makes your eyes feel terrible.
Hair colour is something to look at, too much makeup, too heavy lid colour. I cant wear much colour on my eyes, some silver/white lid cover, black mascara and perhaps a light liner.
I find self tan the best way for making me look healthier and this covers some of the bluey tinge around the eyes too.