Hi lakshya,
dont worry at all .. watermelon does not cause water retention . rather it has a dieuretic effect in the body and helps in flushing out the retained excess water. it is specially good for women during their mesntrual cycle. it helps in lowering the feeling of bloating and fluid retention. not to forget it is one of the best foods you can have if your aim is to lose weight.watermelon juice also cleanses the body of toxic wastes, it also greatly reduces itchiness that result from acidosis. looking at all these benefits , i would definately ask you to have it a fair amount to avail them. watermelon should be avaoided only if the doctor asks you to restrict the pottassium content.