Dark urine even after drinking plenty of water. dehydrated?

Yes, or kidney prob.: Yes, it could be dehydration, but dark urine in the presence of drinking plenty of water could be a sign of kidney dysfunction. If it persists, see your doctor asap.

Possible: Dark urine may be from dehydration as well as blood in the urine or from diet beets redden the urine and b vitamins can make the urine strongly yellow.

Probably: It's probably concentrated which is a sign of being under hydrated. If it persists more than a day despite hydration, see your physician.