Whoever said Qwilfish needs an Evo is full of the purest form of concentrated win. I love Qwilfish, it's been on several of my OU teams and has performed remarkably well. I run. . .
Qwilfish @ Focus Sash
Adamant
6 HP / 252 Atk / 252 Speed
Swift Swim
-Rain Dance
-Waterfall
-Counter
-Explosion
This thing kicks ass. So long as it isn't ruined by SR/spikes, it can come in (usually on the revenge), and then Dance, essentially giving it 500+ speed, or Counter if he's about to be hit by a physical attack, almost always for the OHKO. Double STAB'd Waterfall off of 317 attack is actually very damaging to a lot of opponents, and if he doesn't get a chance to use it, he can always just explode in the enemy's face. Works wonders. I've seen it take out up to three Pokeys before going down.
An evolution with more attack (Preferably 110+ =D) would rock, as it could be a physical Swift Swimmer that isn't Kabutops, with the ability to Explode, D-Bond, Counter, or Spike. What more could you want? =D