Can't help but feel like you guys are heavily overrating Slowbro. It's strong on paper but it has several very good answers (Ampharos, Bellossom, Lanturn, Chansey, Hypno, RestTalk Growth Toed), can be hindered or broken by several common wallbreakers (Thief Magmar, SD Pinsir, SD Scyther, DrumQuag, Politoed, Haunter, Curse/Thunder Weezing, Curse Sudo), and its poor Speed can hinder it against things like Nidoqueen and SD Sandslash, moreso than like, say, Blastoise. There are quite a few ways to deal with Slowbro and most good offenses I've seen can easily fit multiple ways of addressing it.
Slowbro far from invalidates other Water-types. There are plenty of others in the tier and each has something unique to offer: Qwilfish has Spikes, Blastoise is faster than Nidoqueen and Spins, Politoed is a dangerous setup mon, Kabutops is a reasonably fast SD user with a valuable Normal resist, Quagsire is a solid Ground check and can threaten Curse/Drum, Gyarados is a threatening mixed attacker with good Speed, Omastar is an excellent counter to Normals and can Haze. Slowbro's main niches are Thunder Wave, good offensive presence, and high Defense to act as a buffer against Dodrio. It's a great mon but there are plenty of reasons to justify passing on it in favor of its fellow Waters.
Also
Enigami Psychic only has a 10% chance to drop SpD in Gen 2. It was only 30% in RBY.
I can't help but feel like you're heavily underrating slowbro, because most of these don't even deal with it a reasonable amount. Chansey can't do anything back, and will get pp stalled. Hypno can't do anything back, and will get pp stalled. Bellossom doesn't like taking a psychic, and is definitive setup bait for everything after a para, thief magmar/pinsir/scyther/haunter/curse sudo and curse+thunder weezing (huh why thunder) all cannot only not switch into bro, either because of an attack or thunder wave, hampering them for the rest of the game (since you won't (commonly) use a beller with those mons). These are just the mons that bro doesn't outright wall, lol. Yes these are offensive assets you can use the break bro, but they don't do anything about the mon itself, eg you need a free turn, thus bro needs to be forced out. I left out growtoad (heh) and lanturn because these are genuinely good bro answers. I don't see how ampharos is a good repellant though (except on very offensive teams I guess, but honestly ampharos is the single most overrated mon imo, it's like really comparable to ebuzz, and actually ebuzz might be slightly better, yes it's weaker and frailer, but faster and has psychic(/ice punch I guess but it's weaker) which is HUGE, and ampharos doesn't, see next sentence) since nido just switches in on it while ampharos does jack shit to threaten you. Oh and also you'll get worn down by attacks, or get parad, and you kinda don't wanna rest because then your amphy is asleep .__. and it loses important coverage, but it wouldn't work anyway because nido can just recover with moonlight, I'm assuming you're generally going to be using that since your team won't be super offensive since you're using slowbro, and this way you deal with a significant part of bro checks rather well, and guess what then you've given nido a free switch .__. What exactly are you gonna switch in on eq/flamethrower/tbolt? Uuhhhh, Hypno? Guess what, that gets pp stalled by slowbro! Oh and imo ice beam should not be considered, because flamethrower hits not only pinsir but also very importantly piloswine, both of which become much bigger problems otherwise, and what does ice beam hit anyway (gligar was the only mon I could think of).
Yeah that's right, there's exactly 2 slowbro answers in the entire tier! I suppose you could argue curse granbull is also a somewhat decent answer though (then again, only really against sleeping slowbro, and even then you can't get unlucky). Guess this brings up a flaw in my arguing though; bro can't run 5 moves, so you can't have 2atks twave without being much more vulnerable, and without twave you lose an important threat, though you it's not at all impossible to forgo psychic probably. But still, you always need to heavily consider how to beat bro with a combination of offensive pressure and a dedicated answer to it on teams that aren't just all out offence. Curse granbull is much more reliable vs mono attack though, but actually slowbro can even get around that as well as growtoad, a mon you mentioned as a full stop, with a very specific set if it wants (didn't think of this myself, had a discussion with someome who goes by king focuspunch on the ladder who used it, shut both my toad and granbull down cold). Anyway slowbro is just completely ridiculous, not in the slightest because ml nido synergizes so well with it.
Oh and nido, while not being as ban worthy as slowbro probably, is still ridiculous lol, eq/flamethrower/thunder just hits everything. Including slowbro if you manage to hit thunders.
So I guess a list of ways to (consistently) deal with bro would be this I guess (please comment on this if I forgot anything/you disagree): lanturn, growtoad, kadabra/mr mime, thunder nidoqueen (remember: slowking doesn't care), constant boom pressure, constant pressure from SE setup, constant pressure from some of the mons you mentioned (I really did like thief magmar btw), curse granbull sort of maybe sometimes. 5 and 6 of that list being restricted to mostly all-out offensive teams. And on teams that aren't all our offence, you're going to need a more reliable answer. "This Pokemon invalidates the use of a significant amount of viable Pokemon, and because it has only a very select set of mandatory countermeasures it greatly restricts teambuilding, otherwise forcing you to pp stall it do death which leads to very exciting battles" seems like a good tldr on why slowbro shouldn't be in this tier!
psa: I'm not some gsc uu god, not in the slightest, but really anyone should be able to conclude something similar. I don't see a good reason for keeping it in the tier, if people continue to play it after derdomme/it gets an official ladder, I think that whoever has the authority to suspect it/ban it should do so. Also sorry for the messy post, god I hate phone typing.
ps: oops there's more to your post. qwilfish dies to psychic, blastoise can roar it out I guess (btw I think someone said that they think roar stoise should only be used with spikes which makes sense because otherwise is just so passive, but I've never managed to fit both (not even qwilfish on it's own because slowbro/nido electrics and it just literally doesn't do anything except spikes, but of course on very offensive teams it's solid) because they both really don't achieve that much and the best stoise can do anyway is roar it out ._. I guess you can then continue to repel it with offensive pressure, but still, decent answer actually maybe but hopefully you get what I mean, it's still rather passive) and drumtoad can't switch in, and also you'd be relying quite heavily on prediction, and it's just a one time check but I guess which is not that big of a deal on very offensive teams but again, slowbro is going to be spamming twave. Toad definitely isn't invalidated though, completely true. Quag takes a huge amount of damage from from both attacks and is thus not even a good one time answer, slowbro is one of the better kabutops checks, I mean boosted ancient power hurts so you won't be checking a 100% +2 tops but there's no way kabutops can check slowbro. Lastly gyarados does no damage to slowbro and fears twave (unless you're running sub+swagger with twave support, epic idea imotbqh).
I have no idea how you could say slowbro is overrated. I quit oras ubers, and wanted to play a new, fun tier, only to be met with what feels like pdon v0.9 without setup moves and stealth rock. I tend to like centralised tiers, but slowbro is just flat out ridiculous.
tl;dr fuck slowbro
Edit: thanks to
Earthworm for pointing out that I misinterpreted the second half of your post, I guess I was trying to say that slowbro doesn't invalidate all water types, but still manages to often beat them, or at least do well versus them, which makes them harder to use because of how good slowbro is. Oh and actually gyarados 3hkos slowbro with thunder (or rather, has a 0,7^3=34,3% to do so) but slowbro can twave, and psychic always breaks a potential sub. Idek why the standard set has zap cannon, it seems pretty bad.