The simplest performance lever there is — plus a cold-shower trick for firmer skin.
Your body is about 65% water, and even a 6–10% drop disrupts digestion, circulation, and metabolism. A workable daily target is roughly 30–40 g of fluid per kilo of bodyweight — so a 60 kg person needs about 1.8–2.4 litres. Around 60–70% comes from food; top up the rest with plain drinks.
Note that coffee doesn't hydrate you — it actually pulls water out — so if you love it, drink a little extra to compensate. The best choices are still water, mineral water, unsweetened vegetable and fruit juices, and herbal or green tea.
Too little water thickens the blood and tires you out; too much overloads the heart and kidneys — so aim for enough, not endless.
One bonus trick: the contrast shower. Alternating hot and cold water works your muscles almost like a workout, boosts circulation, and firms the skin. In the morning, try about 30 seconds warm then 15 seconds cool for three rounds, building the contrast gradually.