Qwilfish in OU is no joke. Its Attack stat may not be stellar, nor its Speed, but Rain Dance lets it overcome its mediocre Speed stat to become faster than anything commonly used in OU, and the rain helps make Waterfall a truly fearsome attack. Kabutops may have the higher Attack stat, but Qwilfish has many invaluable qualities such as the ability to absorb Toxic Spikes and explode. You may laugh at first sight, but you won’t be laughing when it proceeds to rip apart your team.
Qwilfish makes a great Swords Dance sweeper in the rain; a +2 STAB Waterfall boosted by the rain leaves a massive dent even in Skarmory, doing 76.3% - 90.1% to maximum HP and Defense variants, meaning it can sometimes OHKO with Stealth Rock support. Poison Jab pairs well with Waterfall, hitting most Pokemon who resist Waterfall. It always OHKOes even the most defensive of Celebi at +2, who can otherwise be a pain for Rain Dance teams. Explosion finishes off the set by allowing Qwilfish to go out with a bang and do some major damage.
While many would opt to use Kabutops for the higher Attack stat and a STAB Stone Edge, Qwilfish does have some advantages over the prehistoric arthropod. Possibly its biggest advantage over Kabutops is that it can absorb the Toxic Spikes which cripple most Rain Dance teams. It also has a much easier time against Celebi since Kabutops has to use X-Scissor if it wants to OHKO Celebi at +2, meaning it will lose Aqua Jet and have a harder time against Scizor; on the other hand, Poison Jab is a staple move for Qwilfish. Lastly, Qwilfish has access to Explosion, which OHKOs every non Ghost-type in OU after a Swords Dance.Team Options & Additional Comments >>>
Qwilfish can set up Rain Dance itself, but this is better left to faster or bulkier Pokemon; Qwilfish should stick to Swords Dance. It can also utilize both Spikes and Toxic Spikes, but Forretress does this better in OU. Qwilfish can use Destiny Bond to make an opponent faint if they take out Qwilfish, but you're better off Exploding in OU. Qwilfish has access to Taunt to stop opponents from using status or setting up, but Qwilfish will find very few opportunities to use it. There's Endure/Sub and Flail, but the strong presence of Tyranitar and Hippowdon in OU, as well as priority makes this not worth it. Counter is rather gimmicky, and should only be used with a Focus Sash, but it could surprise some Pokemon. Anything else of note requires Qwilfish to take a hit or be slow (Revenge, Payback, Gyro Ball), which is definitely not acceptable. Aqua Jet could be used as it gives Qwilfish come priority, but that makes it generally outclassed by Kabutops who has a higher Attack stat. Also, Aqua Jet and Swords Dance is an illegal combination, and so this limits Qwilfish's ability to use it effectively in OU.
Just max Attack and Speed. You could drop its Speed down to 264 to just outrun Choice Scarf Gengar in the rain, but you're better off just Speed tying with Kingdra, as the HP EVs would be insignificant.
Checks and Counters
Bulky Water-types are its number one counters. Without a boost, Swampert easily disposes of it with Earthquake, and can even survive if Qwilfish is at +2, although the odds are against it if Stealth Rock is up. Suicune and Vaporeon can take it on if they have Hidden Power Electric or if Qwilfish is weakened, as a +2 Poison Jab only musters a 2HKO. Gyarados is a great counter as it has Intimidate to weaken the attacks of Qwilfish, while being able to take it out with Earthquake, and all Qwilfish can really do is explode.
While they are UU and so aren’t too common in OU, Quagsire and Toxicroak still make great counters as they are immune to Water-type moves because of Water Absorb and Dry Skin respectively, and both resist Poison-type moves. Milotic can do well if it has Hidden Power Electric since it has the bulk to take a +2 Poison Jab. Walrein can do very well against it thanks to its high HP and good Defense, but will need Earthquake to hit back or hail needs to be active; Walrein can then slowly weaken down Qwilfish through Substitute and Protect.