Time for my water analysis! Raikoben did a fine job with several typings, so I wanted to mostly cover a few typings that have been heavily discussed that they didn't - Water/Bug, Water/Steel, Water/Ghost, and Pure Water. I also included my favorite typing (spoiler warning), - Water/Ground, for one more pass.
Note for this I wanted to look at defensive performance – SE and resisted were an afterthought. I also didn't want to do an exact inverse of the "walled on paper" check I did for offense, as it's simply too hard to calculate what we 'wall on paper" - things carry coverage, our stats and abilities are unknown, etc. As a result, we have a new category; “How Bad is that weakness?”. I wanted to know, and hopefully wanted everyone else to know, which 'mons in the meta threaten to crack our defensive shell since we'll need to accept or account for those facts in later stages. To calculate an answer, I asked the following question of each Pokemon rank B- or above:
Is this Pokemon capable of coming in for neutral or resisted damage on our STABs? Does this Pokemon commonly carry a STAB move that is SE against us? If so, they are worth 1 point on the “Weakness” list.
If two of the three criteria above are true, they are half a point. For example, Tomohawk does not get hit SE by Water/Steel and gets Fighting STAB, but it doesn’t always carry a fighting move, so it’s only half counted. Likewise, Kyurem-B doesn’t get STAB on Fusion Bolt, but it pretty much always carries it, so that’s half a point as well. If only one of the criteria is true, they aren’t counted at all – as a common example, Crucibelle often carries Wood Hammer – but Wood Hammer isn’t a STAB move and Crucibelle is weak to water, so she isn’t counted as a weakness unless one of her STABs comes into play.
Water
Resists: 4- Fire, Ice, Steel, Water
Weakness: 2 - Electric, Grass
How Bad Is that Weakness? - Ferrothorn, Jumbao, Tapu Koko, Kartana, Necturna, Krilowatt, Cyclohm, Magnezone, Zapdos, Tapu Bulu, Kyurem-B* (not STAB), Tangrowth, Plasmanta, Venasur-M, Amoongus, Serperior, Rotom-W, Ninetales-A
Hits SE: 3 - Fire, Ground, Rock
Resisted By: 3 - Dragon, Grass, Water
Water/Steel
Immune and Other Considerations: Poison, Sandstorm damage
Resists: 10 - Bug, Dragon, Fairy, Flying, Ice (4x), Normal, Psychic, Rock, Steel, Water (4x)
Weakness: 3 - Electric, Fighting, Ground
How Bad Is That Weakness?– Landorus-T* (Weak to us), Tomohawk* (Doesn’t always run SE move), Tapu Koko, Zygarde, Krilowatt, Cyclohm, Magnezone, Arghonaut, Colossoil* (weak to us), Hawlucha, Medicham-M, Plasmanta, Rotom-W, Gastrodon, Zapdos, Cawmodore* (Not STAB), Swampert-M, Garchomp, Gliscor* (weak to us), Gallade-M, Keldeo, Lopunny-M, Mamoswine* (weak to us), Excadrill* (weak to us), Kerfluffle* (Weak to us), Revenankh
Hits SE: 5 – Fairy, Fire, Ground, Ice, Rock
Resisted By: 1 - Water
Water/Ground:
Immune and other Considerations: Electric, Sandstorm damage
Resists: 4 – Bug, Fire, Rock, Steel
Weak: 1 - Grass (4x)
How Bad Is That Weakness? – Ferrothorn, Jumbao, Necturna, Tapu Bulu, Tangrowth, Venasaur-M, Amoongus. Serperior, Ninetales-A
Hits SE: 6 - Electric, Fire, Ground, Poison, Rock, Steel
Resisted by: Grass types, Flying Dragons (Salamence), Flying Water (Pelipper, Gyarados, Rotom-W)
Water/Ghost:
Immune and Other Considerations: Fighting, Normal, most trapping effects
Resists: 4 - Bug, Fire, Ice, Poison,
Weak: 4 - Dark, Electric, Ghost, Grass
How Bad Is That Weakness? - Ferrothorn, Jumbao, Tapu Koko, Kartana, Necturna* (weak to us), Krilowatt, Cyclohm, Zapdos, Magnezone, Tapu Bulu, Toxapex, Kyurem-B* (not STAB), Tangrowth, Plasmanta, Venasur-M, Amoongus, Serperior, Rotom-W, Ninetales-A, Colossoil* (weak to us), Pajantom* (weak to us), Kitsunoh* (weak to us), Marowak-A* (Weak to us), Revenankh* (weak to us), Sableeye - M* (weak to us), Weavile, Tyranitar* (weak to us), Hoopa-U, Gengar* (weak to us), Gyarados-M, Mimikyu* (weak to us)
Hits SE: 5 - Fire, Ghost, Ground, Psychic, Rock
Resisted by: Dark types with a Water resist (Grass/Dark, Dark/Water, Dark/Dragon)
Water/Bug:
Resists: Fighting, Ground (x4), Ice, Steel, Water
Weak to: Electric, Flying, Rock
How Bad is that Weakness? - Tomohawk, Krilowatt, Cyclohm, Zapdos, Magnezone, Rotom-W, Ninetales-A, Plasmanta, Kyurem-B* (Not STAB), Tyranitar* (weak to us), Stratagem* (weak to us), Crucibelle* (weak to us), Hawlucha, Volkraken* (Not STAB), Pinsir-M, Diancie-M* (weak to us),
Hits SE: 6 - Dark, Fire, Grass, Ground, Psychic, Rock
Resisted By: Lots and lots of dual-typed things with a Water resist, including Water/Fairy, Grass/Fairy, Water/Fighting, Flying/Dragon, Water/Dragon, Ghost/Dragon, Ghost/Grass, Water/Poison, Grass/Poison, Water/Steel, Grass/Steel
Water/Poison
Immune To: Poison status
Resists: 8 - Bug, Fairy, Fighting, Fire, Ice, Poison, Steel, Water
Weak: 3 - Electric, Ground, Psychic
How Bad Is That Weakness? – Landorus-T* (Weak to us), Tapu Koko* (weak to us), Zygarde, Krilowatt, Cyclohm, Magnezone, Colossoil* (weak to us), Plasmanta, Rotom-W, Gastrodon, Zapdos, Kyurem-B* (Not STAB), Swampert-M, Garchomp, Gliscor* (weak to us), , Mamoswine* (weak to us), Excadrill* (weak to us), Tapu Lele* (weak to us), Aurumoth, Latios-M, Latias-M, Alakazam-M, Slowking, Hoopa-U, Jirachi, Gallade-M, Ninetales-A* (weak to us)
Hits SE: 5 - Fairy, Fire, Grass, Ground, Rock
Resisted By: Steel, Ghost, and Poison types with a water resist (Steel/Grass, Steel/Water, Ghost/Grass, Poison/Water, Poison/Grass)
First, let's look at our baseline - Pure Water brings to the table 4 resists and only 2 weaknesses, setting a precedent of a defensive typing that is good bringing us at least twice as many resists as we have weaknesses. In terms of "badness" of its two weaknesses we get a fair number of the usual suspects in the form of all grass and electric types that are viable plus Zekrom, and a nice balanced offensive presence. I think the question every other typing needs to face is does this typing significantly increase our resistances and eliminate our bad matchups? And if it doesn't, does it have overwhelming offensive presence to make up for it?
Water/Steel was a natural one to look at next, as the elephant in the room. With 10 resistances and an immunity contrasted with only 3 weaknesses, Water/Steel is simply unmatched in the sheer number of things we can come in on. However, this is where my analysis is important - despite having only one more weakness and nearly four times as many resists, Water/Steel represents a marked increase on the number of threats to our defensive position. Admittedly many of these are weak to us and aren't going to love coming in on a water or steel STAB move, but it limits our ability to come into them and its something we should be aware of that doesn't make Water/Steel quite the auto-pick insta-win I assumed. Despite that I like it a lot, as it is the best type at taking on M-Crucibelle thanks to being immune to Gunk Shot, resistant to Head Smash, and one of the few typings we can select to only be hit neutrally by Wood Hammer. That's just one 'mon but it is a big one that this typing counters better than any other.
That being said, Water/Ground is my winner. The merits of this typing are well-known, but it finds all things in balance, with the best overall ratio of immunities and resists to weaknesses (4:1), the fewest number of "threats" in the meta since it only needs to worry about the handful of Grass 'mons, and the great offensive pressure such a typing could bring (since we have discussed our Water starter being defensive but still having an offensive presence, as opposed to Toxapex). I think it is inarguably the best type, and the real dilemmas are more ones of process and optics - how can we make potentially two X/Ground types and get that through a vote? And more pointedly, how can we make a Pokemon that fulfills our coordination concept, differentiates itself from Gastrodon, Swampert (itself a starter), and several generations ago star Quagsire while also not being too powerful?
Water/Ghost goes in the opposite direction. It has the worst resist ratio and the largest number of things that can threaten it. That all comes at the value of two immunities instead of just one and a monstrous offensive presence - while it technically hits less things for super-effective damage than some other types, it will have very dangerous neutral coverage. As a less explored typing it offers something very different from the rest of these as an offensive defensive 'mon, and I think it should be slated despite its many deficiencies.
Defensively, water-bug offers a nice middle of the road between Steel and Ground, and despite its mediocre resist to weak ratio it has very little in the meta that can directly threaten it. That 4x ground resistance is also preeettty good especially against Lando and Colossoil. My concern for this typing is its offenses. On paper, it looks like it should be good with six types it can hit SE and only specific dual-types resisting it, but take a moment and think about just how many things in the meta ARE those dual types. This thing is going to get walled a lot, and rocks weakness on a defensive 'mon is also not a thing to celebrate. I don't want to throw this typing out as there are some pros and currently both the Water/Bugs in game are low BST 'mons with bad gimmicks, but there's a lot of risks we'll need to address.
I will be real; I almost forgot to include Water/Poison and had to circle back rather than edit it. It has a lot in common with steel; slightly less resists but a similar profile with plenty of 'mons capable of threatening us on paper while we neutralize our Grass weakness in exchange for introducing a Ground weakness. Like Steel, it also shows the danger of introducing a ground weakness given the prevalence of powerful grounds in the meta. They are offensively similar as well - both are good at killing fairies so its mostly a matter of hitting Ice versus Grass for SE damage and given our defensive nature, I like the sound of hitting ice especially since so many Grass types have a secondary typing that neutralizes their poison weakness.