The short answer to your question is "NO". Thanks to their caffeine content, both tea and coffee are diuretics – that is, they cause the body to lose water. And you do need the water – when you don't get enough, your body cells start to draw water from the bloodstream. Blood gets sludgy, your heart must work harder, and your body starts to redirect blood away from less vital areas.
A Cup of Caffeine Instead of Water?
You don't necessarily have to follow the standard recommendation to drink 6 litres of water per day, but you do need to drink as much of that amount as you comfortably can and usually more than you think you need. This is especially important during and after physical exercise or during hot weather when you've been sweating. Try to drink extra water whenever you think of it – you'll know you're getting enough when you notice that you're urinating more often than usual.
If you really don't like drinking plain water, you can get the fluids you need from highly diluted fruit juice, herbal tea or a well-watered-down sports drink. Avoid caffeine-containing colas – they have the same diuretic effect as coffee and tea.