As much as I love Primarina, I can't this mon being extremely viable in OU. It does have solid typing, good special and enough coverage to boot. Though, it's speed is bad and it's defense is mediocre. The lack of recovery and Calm Mind (Why Game Freak?) means that a more defensive set is out of the water. Also, I'm not keen on Liquid Voice, It's not a good ability IMO. If it boosted Hyper Voice like the -ate abilities, it would be a no brainer pick over Torrent. Since Primarina won't be outspeeding much and has decent bulk, Torrent has a chance to provide some extremely powerful Hydro Pumps also possibly boosted by Specs.
Calm Mind would blow hard on Primarina IMO, especially with Keldeo as a superior Calm-mind offensive threat, and Slowbro as a superior defensive one.
The only thing that makes this thing OU relevant is the Specs set... which is pretty good in my experience. Specs Primarina is a safe-switchin to Heatran (after it uses its Bloom Doom. Just switch to your grass resist to absorb the Z-attack and you're good), Ash-Greninja, Keldeo, Kingdra. The Specs Hydro Pump is extremely useful vs Rain Dance teams as well.
252+ SpA Choice Specs Primarina Hydro Pump vs. 248 HP / 8 SpD Pelipper in the Rain: 288-339 (89.1 - 104.9%) -- 31.3% chance to OHKO
252+ SpA Life Orb Kingdra Hydro Pump vs. 64 HP / 188 SpD Primarina: 116-136 (36.5 - 42.9%) -- guaranteed 3HKO
252 SpA Choice Specs Primarina Moonblast vs. 40 HP / 0 SpD Kingdra: 474-560 (157.4 - 186%) -- guaranteed OHKO
Modest
252+ Sp. Atk / 64HP / 188 SpD
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Hydro Pump
Moonblast
Psychic
Shadow Ball
EDIT: Freaking damage-calculator. Something was wrong. I redid all the calcs... but if somebody notices something extremely wrong... sorry. There seems to be a "rain glitch" that messes up calculators. In any case, I've edited my post, trying to correct the things that were obviously wrong.
Its a good pivot as long as the opponent doesn't have Chansey or Ferrothorn to absorb the damage afterwards. Even resists take big hits.
252+ SpA Choice Specs Primarina Hydro Pump vs. 0 HP / 0 SpD Tapu Bulu: 151-178 (53.7 - 63.3%) -- guaranteed 2HKO after Grassy Terrain recovery
Non-resists take bigger hits.
252+ SpA Choice Specs Primarina Hydro Pump vs. 252 HP / 252+ SpD Celesteela: 207-244 (52 - 61.3%) -- guaranteed 2HKO
252+ SpA Choice Specs Primarina Hydro Pump vs. 252 HP / 252+ SpD Muk-Alola: 210-247 (50.7 - 59.6%) -- 84.8% chance to 2HKO after Black Sludge recovery
252+ SpA Choice Specs Primarina Hydro Pump vs. 252 HP / 16 SpD Assault Vest Muk-Alola: 190-225 (45.8 - 54.3%) -- 54.3% chance to 2HKO
Of course, guess correctly with coverage and the opponent gets into a very tough spot.
252+ SpA Choice Specs Primarina Psychic vs. 248 HP / 144 SpD Venusaur-Mega: 240-284 (66.1 - 78.2%) -- guaranteed 2HKO
252+ SpA Choice Specs Primarina Psychic vs. 252 HP / 252+ SpD Toxapex: 178-210 (58.5 - 69%) -- guaranteed 2HKO
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Considering how often you need something to switch into Ash-Greninja's Hydropump. Primarina is bloody useful for that.
252 SpA Choice Specs Greninja Hydro Pump vs. 64 HP / 188 SpD Primarina: 86-102 (27.1 - 32.1%) -- guaranteed 4HKO
252 SpA Choice Specs Greninja Dark Pulse vs. 64 HP / 188 SpD Primarina: 16-20 (5 - 6.3%) -- possibly the worst move ever
252 SpA Choice Specs Greninja-Ash Hydro Pump vs. 64 HP / 188 SpD Primarina: 114-135 (35.9 - 42.5%) -- guaranteed 3HKO
252+ SpA Choice Specs Primarina Hydro Pump vs. 0 HP / 0 SpD Greninja: 192-226 (67.3 - 79.2%) -- guaranteed 2HKO
Ash-Greninja basically will never get Battle-bond as long as Primarina is switching in. Gunk-shot is Gen6-only. So its a safe time to pivot.
Primarina is a better partner to Mega-Metagross than basically every other water-type in the game. Even Fini, IMO, because Fini is just too passive. Its niche, but its definitely useful within this specific niche. This is definitely a UU-mon at best, but like Ditto or Shedinja... its got applications to the OU-tier because its so unique.