Actually Cocaine is a CNS stimulant on the prohibited list at all times during competition, so for rugby league when the season commences it is banned (however competition time frames do differ on each sport as it is dependant on when competition is, so faor the olympics they are testing well in advance before it commences also TUE's have to be submitted 6 months prior to the start of the olympics). I only know of one case in the nrl where a TUE (therapuetic use exemption) was allowed and it was adam Macdougall, medication he needed to treat an illness where no alternative that wasnt banned was available.
Water also only does so much so people tend to use substances like diuretics to flush the systems. its also one of the reasons why diuretics is also on the banned list.