Let's talk about CAP25w.
I agree that Water/Poison should be avoided. It risks too high of competition with Toxapex. I get the arguments that we COULD make it different, but at the end of the day I think we would just be reinventing the wheel, especially considering CAP25w's BST limitations. We really need to explore how to make the best of the BST without reinventing something, and Water/Poison just gives us Toxapex 2.0 no matter what we do. Toxapex has balanced stats, reliable recovery, a fantastic ability, a solid movepool: everything Water/Poison needs to be successful. I don't see how we can (or why we would want to) create something different enough to be viable. And the arguments for Corrosion and Merciless are irrelevant, since they don't actually rely on Poison-typing STAB and could be used on any Pokemon of any type.
Water/Ground has two viable defensive options in CAP (Gastrodon, who is ranked A-, and Quagsire, who is ranked C), and I think we risk the same problem here as with Water/Poison. Considering our BST limit, I worry that we can't create something that will differentiate itself enough. Gastrodon especially is good in CAP right now, ranking higher than Toxapex in viability, and again, it has what Water/Ground needs to succeed: legit movepool, reliable recovery, great ability, and well-placed stats. No need to create something that already exists.
Water/Steel and Water/Electric are both tricky as defensive mons due to their Ground-type weaknesses, but we have a lot of flexibility to create something that doesn't already exist, and Ground is easy enough to work around with team support. There is no relevant purely DEFENSIVE Pokemon of these types. Naviathan and Krilowatt focus on attacking, Rotom-W needs recovery, and Empoleon and Lanturn are irrelevant in CAP. These types could each be successful, but I don't think we fulfill the brief of CAP25 (ability and movepool coordination) as well as with Water/Grass, which I'll explain later.
Water/Bug is a neat thought, but I'm not sold. Bug has some really cool STAB options that would synergize well with defensive abilities and stats, but the Stealth Rock weakness is the biggest deterrent here for me, which can be crushing for a defensive mon.
Water/Ice, frankly, is a ridiculous typing for a defensive mon. Being weak to Stealth Rock already sucks, and Ice does nothing for this typing defensively other than adding a x4 Ice resistance, which is not a worthy trade for the additional weaknesses to Fighting and Rock and losing resistances to Fire and Steel. Aurora Veil is not enough justification to make this seem viable to me. CAP25w would have to be fast AND bulky to make that work, and I don't think we can make that happen with our starter BST limit.
Water/Dark should also be avoided. The weaknesses are too common. Losing to Fighting, Fairy, Bug, Electric, and Grass is just too much for a defensive mon. And, as it has been pointed out, with our BST limits as a starter, we can't be as bulky as we would need to be AND hit as hard as we would need to with Pursuit, Knock Off, Sucker Punch, etc.
As of now, I think I'm most convinced with Water/Grass for CAP25w. Ground-resistance is awesome for a defensive mon; being neutral to Electric is great for a Water-type; Bug, Flying, and Poison are easy enough to work around; and we get some really awesome defensive movepool / ability synergy. Grass-type moves have an abundance of ability-supported defensive options, and Water is a great defensive typing. Plus, this type combo doesn't risk being overshadowed by anything else (no one uses Ludicolo, especially defensively.)
I agree that Water/Poison should be avoided. It risks too high of competition with Toxapex. I get the arguments that we COULD make it different, but at the end of the day I think we would just be reinventing the wheel, especially considering CAP25w's BST limitations. We really need to explore how to make the best of the BST without reinventing something, and Water/Poison just gives us Toxapex 2.0 no matter what we do. Toxapex has balanced stats, reliable recovery, a fantastic ability, a solid movepool: everything Water/Poison needs to be successful. I don't see how we can (or why we would want to) create something different enough to be viable. And the arguments for Corrosion and Merciless are irrelevant, since they don't actually rely on Poison-typing STAB and could be used on any Pokemon of any type.
Water/Ground has two viable defensive options in CAP (Gastrodon, who is ranked A-, and Quagsire, who is ranked C), and I think we risk the same problem here as with Water/Poison. Considering our BST limit, I worry that we can't create something that will differentiate itself enough. Gastrodon especially is good in CAP right now, ranking higher than Toxapex in viability, and again, it has what Water/Ground needs to succeed: legit movepool, reliable recovery, great ability, and well-placed stats. No need to create something that already exists.
Water/Steel and Water/Electric are both tricky as defensive mons due to their Ground-type weaknesses, but we have a lot of flexibility to create something that doesn't already exist, and Ground is easy enough to work around with team support. There is no relevant purely DEFENSIVE Pokemon of these types. Naviathan and Krilowatt focus on attacking, Rotom-W needs recovery, and Empoleon and Lanturn are irrelevant in CAP. These types could each be successful, but I don't think we fulfill the brief of CAP25 (ability and movepool coordination) as well as with Water/Grass, which I'll explain later.
Water/Bug is a neat thought, but I'm not sold. Bug has some really cool STAB options that would synergize well with defensive abilities and stats, but the Stealth Rock weakness is the biggest deterrent here for me, which can be crushing for a defensive mon.
Water/Ice, frankly, is a ridiculous typing for a defensive mon. Being weak to Stealth Rock already sucks, and Ice does nothing for this typing defensively other than adding a x4 Ice resistance, which is not a worthy trade for the additional weaknesses to Fighting and Rock and losing resistances to Fire and Steel. Aurora Veil is not enough justification to make this seem viable to me. CAP25w would have to be fast AND bulky to make that work, and I don't think we can make that happen with our starter BST limit.
Water/Dark should also be avoided. The weaknesses are too common. Losing to Fighting, Fairy, Bug, Electric, and Grass is just too much for a defensive mon. And, as it has been pointed out, with our BST limits as a starter, we can't be as bulky as we would need to be AND hit as hard as we would need to with Pursuit, Knock Off, Sucker Punch, etc.
As of now, I think I'm most convinced with Water/Grass for CAP25w. Ground-resistance is awesome for a defensive mon; being neutral to Electric is great for a Water-type; Bug, Flying, and Poison are easy enough to work around; and we get some really awesome defensive movepool / ability synergy. Grass-type moves have an abundance of ability-supported defensive options, and Water is a great defensive typing. Plus, this type combo doesn't risk being overshadowed by anything else (no one uses Ludicolo, especially defensively.)