The legal drinking/purchasing age of alcoholic beverages is 18.
All tap water in Mozambique should be assumed to be unsafe to drink, even if it is not harmful it usually has some sediment that your stomach will not be used to. Most western oriented lodgings either provide a fresh water source or sell bottled water.
In Mozambique Cervejas de Mocambique who are owned by SABMiller (http://www.sabmiller.com/...) have a virtual monopoly on beer brewing. The three most popular brands are 2M remember to pronounce it doysh-em or you will end up with an extra beer, Laurentina Clara and Manica. Other local African beers such as Castle and Windhoek are reasonably widely available but are not as popular as in neighboring countries due to the high quality of the local brews.