Is Sharpedo Viable?

I'm inclined to say, "no". It's doubtful that it will get off a hydropump for water attack. Ttar, houndoom, and absol are all better choices for dark attack.

if it gets a decent buff from the rework it might be decent

Sharpedo lasts longer against Mewtwo than Gengar does.

It is actually good if you want to do a unique 6 tier 3 solo against the likes of claydol or starmie. Otherwise, not much use.

It has Gyarados' Waterfall + Hydro Pump moveset and with a slightly greater Attack stat. So for gym clearing when you're wouldn't faint, say against a sole Golem or Rhydon, it would clear them faster than Gyarados.

The issue being that because its bulk is so much worse than Gyarados, with only a small increase in Attack, Gyarados actually has more DPS than Sharpedo in almost all scenarios. Gyarados will take much longer to faint and hence less time spent on the fainting animation, even against a raid Rock Throw / Rock Blast/SE Golem. Unsure if Sharpedo's DPS will be lower against Golem after the CP rework though - Sharpedo is relatively one of the best Pokemon to benefit from it with its low HP stat and not expecting to lose Def, so Sharpedo should experience an increase in DPS (and TDO) then.

Viable? Hell no!!!! Fun to battle with? Hell yes!!!!

Its DPS is very good but it's so frail it's really only useful if you do unique 6 solos for the challenge. Would not recommend for general raid use, I have one maxed and plenty of experience using it in solos.

tl;dr Not viable.