none of the Starters are outright useless into him
uh
Linoone having Mud Sport to give it a fake Resistance and compatibility with Rock Smash to break Magneton who resists their strong-for-the-moment Headbutt (and you might be carrying as an HM user or Pick Up bot even if not a major battler)
uhhhh
Nincada as a Ground or a potential Shedinja (depending on if the player uses "filler" slots or not by the time it evolves), the latter of which literally can't be hit by anything besides Voltorb's Rollout
uhhhhhhhhh
Okay I guess I wouldn't say it's completely useless, but Grovyle is so ass against Wattson. It might take a few turns to fall thanks to its Electric resistance, but its incredibly weak moves relegate it to Screech support against the major threat in Magneton (assuming you've raised it to Level 23). It can't even reliably beat his Voltorb, particularly in RS where it has Sonicboom.
Linoone gets Mud Sport at Level 23 (or 21 as a Zigzagoon if you delay evolution) so I think we can discount the idea that you'd just incidentally have an HM friend/Pickup pal Linoone with Mud Sport for Wattson. Even setting that aside, though, a same-level Linoone's Rock Smash is, under the best possible circumstances (excellent IV, +Atk nature, Defense drop procs every time, universally high damage rolls, no hax from TWave or Supersonic) a 4HKO on Magneton, whereas Shock Wave, even under Mud Sport against a Linoone with exceptional HP/SpD IVs, is a likely 5HKO. This is a one-in-a-million scenario I've constructed and you still just barely come out on top.
Shedinja fears more than just Voltorb's Rollout, with Magnemite (RS) and Magneton each knowing Supersonic. Both naturally outspeed you and the AI understands Wonder Guard (except when it comes to switch logic), so they'll either use Thunder Wave then Supersonic or immediately go for Supersonic, at which point you'll instantly have a 27.5% chance of self-KOing. Meanwhile, you're poking away with Fury Swipes or Scratch. Sand Attack can maybe buy you a few turns, but unless the rest of your team is also resistant to Shock Wave it doesn't function super well as team support.
I agree that the abundance of optional trainers nearby make Wattson way more manageable, and there are definitely more viable options than it initially appears, but a lot of the quirkier ones either only put in work against the mons that aren't really a threat anyway or rely on specific combinations with other Pokemon that could probably manage fine on their own with a couple of extra levels.