A functionalist sociologist looks at the ways in which the different parts of society are interconnected and rely on each other. In their view, one part of society fills a role that helps the operation of the society as a whole. A functionalist sociologist might look at the interrelatedness or interconnectedness of the water to the glass (and to the faucet, etc). In other words, a functionalist would understand that several components go into creating the glass of water.
A conflict theorist examines the tensions and stresses that result from the unequal distribution of resources in society and looks at these tensions as catalysts for social change. A conflict theorist might look at the full part of the glass as causing tensions for the empty part of the glass.
The interactionist theory looks at the symbolic meanings people attach to everyday interactions between people. In other words, interactions between people are framed by the symbolic meanings we attach to them. A person from this school of thought might look at what the glass of water symbolizes. For example, is it in a fancy Evian bottle? That type of glass of water would symbolize a world of wealth and privilege.
The postmodern theory stresses the declining importance of once-formative institutions such as the state, church, marriage, school, etc. In an increasingly global and media-driven society, each individual is free to choose his, her, or their own lifestyle. A postmodern sociologist might decide that whatever way the person wants to look at the half-full glass is up to that person's own taste and values.