Sweet tea maybe.. I've never heard "sweet water" but webster does have this,

https://www.merriam-webster.com/dictionary/sweet%20water

> : a sugar solution

> especially : one obtained by recovery of waste sugar during refining

I would not say it's common: I'm a native (American) English speaker and I've never heard the word before! I would just guess it's some kind of flavored water before reading this thread.