What it does
Hydro Cannon is a Water-type special move sitting in the same tier as Hyper Beam, Frenzy Plant, and Blast Burn — enormous power, one-turn recharge. After you fire it, you cannot move or switch the following turn. That recharge turn is the entire cost of admission.
In doubles, that cost is brutal. Your opponent has two Pokémon that can act freely while yours sits still. They can swap, set up, redirect — whatever they need. Unless you're securing a guaranteed KO AND the board state afterward is still in your favor, Moonblast or Surf will almost always be the right call instead.
Who runs it
Primarina is the Champions-format user here at 6% usage. High Special Attack, Water/Fairy typing, and access to more reliable Water and Fairy attacks in Surf and Moonblast. The low adoption rate reflects how risky the recharge is in doubles — most Primarina builds skip it entirely.
How to use it
Before clicking Hydro Cannon, ask one question: what can my opponent do to me next turn? If the answer is 'nothing dangerous,' fire away. If the answer involves free setup, a free switch, or a threatening attack landing on your now-helpless Primarina, you're probably better off with a safer option.
The cleanest window is endgame cleanup — opponent is down to one Pokémon, your side is mostly intact, and one big hit closes it out. Recharge into a dead board is cheap.
Tips & strategy
Protect on the recharge turn neutralizes the damage exposure, but it requires you to correctly predict the opponent is coming for Primarina specifically. If they pivot or set up instead, the Protect is wasted and you've lost momentum anyway.
The move's real value might be as a threat rather than a routine click. Opponents who respect the potential Hydro Cannon sometimes play more conservatively — that caution can create openings for your partner even if you never actually use it.