Yemen is officially a dry country, however non-Muslims are entitled to bring up to two bottles of any alcoholic beverage into the country. These may be drunk only on private property, and venturing outside while under the influence is not a wise decision.
Many juices and soft drinks are readily available, but you should avoid more scruffy-looking juice shops as they might be using tap water as base. Many Yemenis will drink tea shay or coffee qahwa or bun with their meals. Yemeni coffee is considerably weaker than the strong Turkish coffee found elsewhere in peninsular Arabia.
Tap water should be avoided. This is comparatively easy to do, as bottled water - both chilled and at room temperature - is readily available everywhere.