Where's The Still Water?
There's a stereotype that Germans, and Europeans more generally, are super into sparkling spring water. It's also said that you can't get tap water at German restaurants. Both of these are 1010% fact. Mineral water has been and continues to be a historical and cultural fixture of German life, so what is a non-fizzy water drinking person to do?
In the U.S., it's common to see people regularly drinking bottled water, particularly smaller bottles in the 16-20 oz range. In Germany, you can still buy these bottles, but due to the deposits on individual bottles (25 cents for the single use ones!), they are usually prohibitively expensive to purchase in the quantities you might get in the U.S.. Instead, it's far more common to see people carrying around bottles in the 1-1.5 liter bottles of, well most drinks actually.
If you are at a grocery store or restaurant, you will usually see bottles labeled Still (sometimes also "Naturelle"), Medium, and Classic or some variation on those themes (Medium is typically the consistent one). Depending on the brand, the color of label and cap can help you distinguish them, with red or pink being for still water, green for medium, and blue for the full carbonated.
Double check the label though if you're looking for one of them specifically, some brands like to be "different" and can throw you off if you're getting one you wouldn't typically. Should you still not be sure, look for "ohne Kohlensaure" (without carbonation) or "mit Kohlensaure" (with carbonation), which will clear up any confusion from the branding. You may also see or hear it referred to as "Sprudelwasser" or "Wasser mit Gas" which mean fizzy water.
Bottled water in Germany, unlike what often happens in the U.S., is always mineral water, and they're proud of it. In restaurants, you'll often only be able to get mineral water (at a minimum still or full carbonation) in 0.2 or 0.75 liter bottles (no ice cubes!). You can try asking for "Leitungswasser" (tap water), but depending on the restaurant (or more realistically, if you have already ordered some other kind of drink or meal too), they may refuse to give it to you.