This is probably the cheapest (15 Lari a day) option in Tbilisi for escaping the summer heat. 2 large pools of Tbilisi Sea Club get water supply from Tbilisi reservoir itself. In fact this facility is simply the best organised corner or a peninsula at so called Tbilisi Sea. A small pool for children and a toboggan are also available. Admission for children younger than 6 is free.
