1. Not having enough water. The body eventually needs to hold on to water because it does not have enough for the bodily functions. This is because the person did not drink enough water, or ate too much dehydrating food or sweat out too much water and did not make up for it.
2. Drinking too much water. The body and kidneys can only handle so much water. If a person drinks more than that, the body cannot get rid of it effectively. In the end the water will end up around the ankles, hands and abdomen, making the person look a bit fat, bloated and as if having a potbelly. This Ultracycling - Drinking Too Much article explains more about this.
Below is a youtube video about water retention. It says Men Don't Watch but what the hell! Haha