There is no Water type gym leader in Pokemon black. that is unless you chose tepig as your starter pokemon and are talking about cress. to beat cress, if you go to the Dream yard to the east of straition city, in the dream yard there is a lady to the far right, if you talk to her she will give you a pokemon that helps you agents the gym leader. if you chose snivy you get a panpour. oshiwott-pansear, tepig-pansage. take the pansage, level it up on the route before the city to at least Lv 15, defeat the gymleader lilpup with your starter, and then defeat his panpour with your pansage. Spam the move vine whip and its a win.
hope it was helpful~
edit: I put the wrong direction, i meant east
Have a water ice or grass pokemon.
Water pokemon
Beat the Gim Leader.
To defeat Clay you must use water type Pokemon, cause he uses ground type.
beat the gym leader
water or grass type Pokemon
battle wild pokemon until yours is lvl18+
To defeat or "beat" the 7th gym leader in Pokemon sapphire there twins and they have a solrock and a lunatone to beat them I suggest about level 40-50 water types.
8th gym leader
Water, dark and grass Pokemon
Use pansear you get it in the dreamyard
In the town where Ltg. Serge is a gym leader there is an officer there with a squirtle. Beat the gym leader and talk to her afterwards.