Yes, you could lose the weight.
Yes take water to the Gym.
No no specific amount of daily water intake really, just opt for about 2-3 glasses minimum a day, if you want ot drink more, remembering you'll get quite a lot of water from food anyway.
As for the Back see a Chiropractor, as it could be a bit of Lordosis, (lower Back inward curvature), Kyphosis, (Upper Back outward curvature), or Scoliosis, (slight lean over to one side).
You could then be prescribed possible exercises, bases on what is what. Sound more like some Kyphosis, being a slight issue with some of the Thoracic Vertebrae, between the Cervical, (Neck), and Lumbar, (Lower Back). The Thoracic having 12 Vertebrae in total.