Your body is approximately 76 % water !
Your brain is approximately 85 % water !

For every measure of tea, coffee or alcohol, you would need to intake the equivalent measure of water to maintain hydration, in addition to your 2-2½ litres.

Dehydration can result in; aging, poor kidney function, dry skin, asthma, allergies, hay-fever, snoring, sore joints, poor digestion, congestion, reflux, hypertension (high blood pressure), water retention, lethargy, poor concentration, muscle cramps/spasms, arthritis, reduction in fat loss, cravings, etc.

When you exercise you will likely lose more fluid than being sedentary, and if the climate is hot, more again.
The guidelines for normal regular active human would be 2.5 litres of water per day, and would require the addition of more fluid if; exercising, hot climate, drinking tea coffee and/or alcohol.
A good way to measure fluid requirement for post exercise, would be to weigh yourself before the exercise session, exercise and weigh again, replace what you have lost.

Please don't fall into the trap of taking water with you on a run, unless you are running for more than 60mins you should be ok with the fluid you have taken in before the activity. Some people do not exercise well when taking on board fluid, so leave until 30 mins after the exercise session and hydrate then.

This is all assuming that you are already hydrated

REDUCING BLOOD PRESSURE

If you are dehydrated this will make your blood thicker than normal, and therefore increase the pressure through the arteries and veins.

You are advised to drink 3 glasses of water upon waking.

Eat 3 times per day, as per recommended foods article.

Only drink water between meals, leave 30-60 mins either side of a meal.
Do not drink water with a meal.

Try and balance each meal so that it’s about the same size.

It will take approx 3 months of constantly applying the above, to reduce your blood pressure.

A lot more information can be found in the following book ; ‘Your Body’s Many Cries For Water’ by Dr F. Batmanghelidj, MD