You forgot about rotom-w...................
Indeed I did. Will post calcs to amend that.
0 SpA Rotom-W Hydro Pump vs. 0 HP / 0 SpD Manectric in Rain: 280-330 (99.6 - 117.4%) -- 93.8% chance to OHKO
252 SpA Manectric Thunderbolt vs. 252 HP / 0 SpD Rotom-W: 120-142 (39.4 - 46.7%) -- guaranteed 3HKO after Leftovers recovery
0 SpA Rotom-W Hydro Pump vs. 104 HP / 0 SpD Swampert in Rain: 171-202 (46.5 - 55%) -- 67.6% chance to 2HKO
252+ Atk Swampert Waterfall vs. 252 HP / 212+ Def Rotom-W in Rain: 68-81 (22.3 - 26.6%) -- possible 5HKO after Leftovers recovery
Swampert cannot handle Rotom-W, risking a potential burn. That said, Manectric, the competition for Electric solution I was analyzing doesn't fare much better, requiring essentially no prior damage to be OHKO'd while managing only a 3HKO of his own, so he can't even check a healthy one. That said, of the 3 I checked, Swampert wins against 2 (Thundurus and opposing Mega Manectric) by a wide margin, and the last one, though Manectric works better, neither really is going to win against the Washing Machine. That said, I would personally penalize Swampert less for that than I do Manectric for the following reason: Swift Swimmers are supposed to generally be a Rain Team's sweepers, and like any win condition, some checks or counters typically need to be removed first, and I find it hard to believe a Rain Team has no way to deal with what is simultaneously the most used Water (Greninja discounted for Protean) and Electric Type in the tier, when those are arguably two of the most crucial types to prepare for already. Manectric (in the scenario described)
IS supposed to be that solution to opposing Electrics, and he fails against Rotom-W while being lackluster for the others.
In the scenario I've been working under, the argument is that Swampert is a redundant Swift Swimmer because the Mega Slot could better go to something else to handle problems to rain teams like Electrics. However, Swampert is ultimately having little trouble with 2 out of the 3 most prevalent, while Manectric is outperformed against those 2 and underwhelming against the 3rd anyway. I go back to the Sceptile comparison because, while Sceptile has poorer match-ups against those 2 due to HP Ice, he clearly hard counters Rotom-W, and thus might be worth the team slot if your team really struggles with it and can already answer the other 2.
Swampert interests me because he naturally overcomes some of the typical problems of a SS, those being fraility, weakness to priority, and problems with Thunder Wave/(Some) Electrics. He does have the Mega Slot as part of the opportunity cost, but he also requires a bit less support/fewer threats worn out to compensate. Same as Kabutops, he has Low Kick for Ferrothorn, which is also a common headache for Rain teams. Even if Swamp only gets the one kick, Ferro's lack of reliable recovery means it's easy for another member to finish the job.
252+ Atk Swampert Low Kick (100 BP) vs. 252 HP / 88+ Def Ferrothorn: 180-212 (51.1 - 60.2%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Now, I'm not arguing Swampert as more viable, hence staying in B+. However,
as Rain's check to Electric Type threats, Swampert outperforms Manectric in every relevant case, and Sceptile deals with a different one, if more common one. I won't pretend he brings the same raw power as Heracross, but Swampert offers a Swift Swimmer that deals decently enough with other threats, so the things Hera would deal with can reasonably be handled by something else easily enough.