Every art piece has an artistic message. And im given some topics for my art exam and i chose 'Water's Edge'. Can anyone tell me how i should develop a great Idea or message for my final art piece?

Science gives knowledge and art gives inspiration. Study the medieval paintings; they will give you inspiration. http://www.search.com/search?q=medieval+paintings+gallery

If you pay a lot of attention to the news, then you want to include that in your prep work. Issues related to poultion, thirst, flooding and other natural disasters. You may also want to look in books of mythology and find out about how various ancient world cultures depicted their gods and creatures of the rivers and oceans. For example, many rivers in Greece were actually named after the god of that river. There is also the story of Andromeda who was chained to a rock as a sacrifice to a sea monster. That myth has been the subject of many classical artists. You may want to combine modern political and social issues with old world beliefs that you feel relate to "water's edge". Also, listen to some songs and make use any lyrics that you feel give a similar message to what you are after. These are just a few places I would start :)