A Flight Surgeon once told me that if you are accustomed to exercise that one way to ensure a good reading is to get a good workout an hour or so before your physical. That obviously will raise your BP but after you are done it falls significantly for a while.
Drinking a lot of water would actually increase your blood pressure. Some patients with high BP are may be given a medication called lasix aka "water pill" (if diet and exercise does not work). Lasix is a diuretic to lower the BP by allowing the kidney to get rid of excess fluids in the body via urination.