I think that shooting on a white, bright, well lit background would be the best way to illustrate a glass being half full. To add to the optimism, I think that making the glass of water seem more like a tropical drink than just a plain glass of water might be an interesting route. People seem to be happiest in the summertime so making the drink feel summery using food coloring, fruits or an interesting straw in a cool glass (like a mason jar or a really tall, thin glass) would be a good idea. Multiple glasses of different sizes would be interesting as well to show the versatility of having this optimistic outlook in any situation.