Each of us believes it to be a duty to refresh oneself on a hot day by jumping into some water body, but we should be careful here. The enquiry is that swimming in chilly water on a hot day is rather dangerous, especially if you stayed on the sun for quite a bit before going for a swim. The thing is that once overexposed to sun you can provoke spasms in the heart vessels. Therefore, you should cool down in the shadow a bit at first and only then rush into water.
Regarding drinks, besides juice, tea, and soup you should drink at least 1.5 litres of water on hot days. Mineral water works great here. On the other hand, stay away from strong tea and coffee when it is hot outside. The caffeine found in those drinks only enhances water loss.
A small reminder that filtration of the water form the tap does not guarantee its disinfection. Therefore you should boil water and keep the bottled water in the fridge (once you opened it).