A specially defensive Azumarill set with Sap Sipper - sounds silly in the context of BW OU, but remember that this is monotype, and pokemon often have to take on odd roles - walls Celebi fairly thoroughly, but generally fails at doing anything in return - He can't use Encore and Ice Punch on the same set, for example, and Celebi couldn't care less if he was Toxic'ed. Encore does allow an immune partner to set up on an Earth Power / Psychic however, and allows anyone to set up on a Nasty Plot, and a +1 Return with no attack investment does do some damage, as does Ice Beam, though not enough to defeat Celebi generally.
Something like 252 HP / 252 SpD / 4 SpA Sassy is a decent set for walling grass types, since they rarely carry physical attacks (and those that do mostly run mixed sets where the grass attack is the main physical attack, such as Venusaur or Tangrowth). Choosing moves is somewhat difficult though - Toxic seems important for consistent damage, and Encore is pretty central. Ice Beam helps to beat a lot of grass types, and Waterfall and Return can take advantage of the Sap Sipper attack boosts somewhat. Then there's always Protect.
I think Mantine could work as a Celebi check, being neutral to Grass and immune to Earth Power. It struggles to do much else other than that, though.
Mantine can't beat Celebi without signal beam.
Modest 252 Mantine Signal Beam does 90% at best to 0 HP/0 SDef Celebi lol.
4x still isn't enough. Poor mantine.
Hydration Vaporeon and Alomomola make great walls and healers for Water Stall. Tentacruel can lay down Toxic Spikes and spin. Jellicent is a great spin blocker. RestTalk Gyrados can wear down enemies with Dragon Tail. Ludicolo can stall out lots of things using LeechSeed+Rain Dish. Cloyster has access to Spikes, while Swampert has Stealth Rock. Rotom-w makes a good tick user, which can kill some of Stalls bigger threats (Reuniclus being the big one). Rain Stall with just waters is entirely possible, and could even turn out to be pretty good.I don't think stall is going to happen with ANY mono-type team... theres just no way to consistently cover your weaknesses. Stall is based on the idea that you can switch in somebody to everything consistently... but being only water type, its unlikely we will be able to keep a core group healthy long enough to make stall viable.