No Slowpoke has never been able to evolve with a Water Stone.
Slowpoke evolves into Slowbro at Level 37 or into Slowking when traded holding a King's Rock.
The Pokémon that a Water Stone causes to evolve are:
Poliwhirl into Poliwrath
Shellder into Cloyster
Staryu into Starmie
Eevee into Vaporeon
Lombre into Ludicolo
Panpour into Simipour
Hope I helped :)
Yeah, your just crazy. Slowpoke evolves into Slowking via trade holding Kings rock and evolves into slow or at Lv. 37