Agua sin Gas?

For some people, sin is their favorite four-letter-word. But what does sin have to do with water in Peru?

When traveling abroad, you are always warned to avoid drinking the local water because of risk of gastric distress (also known as Montezuma's Revenge, or less politely "the runs".) When I was preparing for my trip to Latin America, the stats I saw were that up to 40% of travelers suffered from some distress and around 30% had to limit their activities because of it. So I resolved to follow the recommendations and always drink bottled water.
But wait, every Spanish class I've ever taken teaches you the phrases like Necesito un vaso de agua, or
I need a glass of water.  Never in my life have I heard someone explain how to ask for bottled water in Spanish.
Even reading the bottle doesn't help. I know what agua is. And sin, in Spanish, simply means without. But what does gas mean? I pulled out my Spanish-English dictionary, looked it up, and found that gas, in Spanish, means... gas.  A lot of help that was.  Water without gas.
It wasn't until I was shopping a few days later that I figured it out. While reaching for my agua sin gas, I noticed beside it a nearly identical bottle of agua con gas. Then it hit me - gas is carbonation! So if I want a bottle of carbonated water, I ask for Agua con gas, por favor. And if I want just bottled water, I ask for Agua sin gas, por favor.