you need it to keep you hydrated and stops your cells from shrinking.
water is used for hydrolysis reactions in the body- digestion which is essential for glucose formation for aerobic respiration, if it has to use anerobic respiration, it can lead to lactic acidosis
also you need water to stop you from having hypotension (low blood pressure)
less water in the blood means that there will be a lower hematocrit and according to virvhow's triad (hypercoagulability, stasis, reduction in blood flow (stasis)), it will make a thrombus more likely
water also plays a key role in internal body temperature regulation