I WOULD SUGGEST A HIGH PROTEIN, HIGH CALORIE MILKSHAKE 3 TIMES A DAY, PLUS SOME POWERLIFTING WORKOUTS 3 DAYS A WEEK. GNC HAS SOME VERY GOOD WEIGHT GAIN MIX POWDERS FOR SHAKES.
You probably have a very high metabolism which makes it hard to gain weight. I would slowly start to rasie your calories are start a weight training program. Make sure you are eating quality foods,...
Water contains no calories (pure water) so if you only drink water the only weight you gain will be temporary (the water resting in your body until evacuation), and no fat will build do to zero...
The usual and most often correct answer is that less calories are being burned off then are being taken in. There are other possible conditions that your Doctor may be able to help you with.