Small sea or big lake officially known as Tbilisi Reservoir is an artificial lake opened in 1953. It's the largest lake around Tbilisi and is very popular during hot summer days when standing in the city is getting unendurable.
Tbilisi sea is the most popular destination for swimming outside even thou is not very clean like any other lakes around. The beach is free and often crowded. Check out the Temka Suburb side and have a swim or get some sunbath.
One of the ways to get to the beach is a taxi from Sarajishvili Metro station.