Or you could, you know... just use something retarded like Life Orb Modest Gorebyss with Hydro Pump, which does 70% minimum in 2 hits with the rain boost, and that's to MAX HP / MAX SDEF CAREFUL Nattorei. Brave or Impish sets not running Special Defense are straight up 2HKOed by Rain Hydro Pump with a layer of Spikes and have a good chance of KO with Stealth Rock as well. It also has the extra added bonus of basically absolutely obliterating anything that doesn't resist it, while Super Fang is just vaguely annoying.
If you really want to beat Nattorei on a rain team, just hit it in the face with boosted Surfs and Hydro Pumps all day long. Don't fiddle around with silliness like Super Fang when you could just be KILLING EVERYTHING.
It's not that easy because theoretically, you should be winning 100% off your matches this way, save for other Rain teams. In reality, you're probably dealing with smart player who predicts your Surfs and switches to a Water Absorb Pokemon.
You'd have to sacrifice Gorebyss to get those two hits in, which might not happen because of Hydro Pump's 80% accuracy. Why sacrifice Gorebyss for the rest of the team when you could Shell Smash and win the game?
The benefit to Super Fang is that it's an indiscriminate 50% off of anything, regardless of what switches in (save for Ghosts, but Huntail learns Crunch). Coming from a both offensively and defensively potent Pokemon, it could be a really huge problem and pave way for a Gorebyss sweep.
Players are starting to adapt to "blindly attack everything" and take advantage of it by predicting well and turning tables on the momentum. Sub/Super Fang Huntail is much more versatile and pressures your opponent into choosing what he wants to weaken or lose, knowing that he has other Swift Swim sweepers to deal with afterward. You can't Toxic, you can't Twave, and he never goes down doing less than 50% to anything (even in situations where it'd be smarter to, you know, attack).
On his own, Huntail is basically a physical Gorebyss, which isn't bad at all, but he becomes much better when you use him as sweeper support so that the real monsters can get in and do some damage. Don't forget he has Shell Break too, and the only thing I can really see stopping that cold in its tracks is Nattorei.