Keep the good carbs up if you want to put on weight (bread, pasta, everything that is yummy but not saturated fat). The water is great for you but wont actually help you gain weight in the short term (apart from an immediate result which is gone within a day). If anything you can actually lose a marginal amount of weight by drinking a lot of water as it improves the efficiency of your body in processing your food (which is a good thing for sure).

And don't stress about BMI. It's a guide and is great if you fit a certain narrow profile. It's good for showing when people are overweight, but pretty useless for people who have a slight frame and naturally don't carry a lot of weight. DW is always on the fringe of underweight and average and she is perfectly healthy, just small.