Drinking alot of water will increase your blood pressure true but you will also lose alot of sodium while perspiring. While you lose more sodium, you also tend to lose more water. This may increase blood osmolarity and therefore lower blood pressure. This will also depend on the temperature and why you are perspiring. With high temp. you lose more solutes and water. Thus lowering your BP. In the winter you wouldn't sweat much. Sweating is a means for compensating for the neg balance generated by water and thus hi blood pressure. Losing more sodium may be helpful, but it doesn't solve the problem since water follows causing no net change.