Water retention is the fluid your body stores. Racing jockeys have their doctors prescribe them something to rid them of excess water, but his only helps them for the short term. (May lose a pound for the moments) It does not put on weight, and in taking a tablet, you will only get the same fluid retention later. You're better off just living with it than trying to rid yourself of it, as it is natural, and not fattening.
You could be dehydrated or eating foods that contain too much sodium. Mainly processed or fast foods have a lot of sodium. Examples of foods with too much sodium are Oriental noodles or children's lunch snacks or frozen dinners even the so called healthier versions. Try drinking distilled water and you can get a scale that measures your hydration, it should be above 52 anything below is dehydration, since our body is made up mostly of water.
This is primarily an issue for women around the time of their period. The body keeps excess water on-board and can make the person FEEL like they are "fatter" But, usually they don't look noticeably "fatter" to others.
Water retention is not fat it will only make people feel bloated a little some people will take duretic tablets but in all honesty you can achieve the same results by drinking 2 litres of water a day to improve your kidney function which will then release excess water