I oppose Dry Skin and Drought on principle, and upon review of the concept.
Drought does indeed bolster both abilities, and would almost certainly make the CaP viable. However, I feel as if this might be a little bit too much. If the concept was "create the perfect typing for a drought mon" I would fully support it, due to its immunities to damaging status. However, the concept is "find a way to bolster the typing so that the typing becomes its stregnth." Let's be honest. How many people would use a Pokemon because it has immunities to damaging status, oh, and it happens to have Drought? How many more would use it because it has drought, and happens to have immunities to damaging status?
Exactly. Drought would overshadow the value of the typing when building a team and picking CaP3. Beyond this, it would be no overstatement to call Drought one of the greatest abilities out there. And I think that Krillowatt taught us what can happen when we randomly apply a powerful ability, when other options exist.
Dry Skin also gives me problems, as I feel as if people are using it to justify using poison STAB offensively. Realistically, the concept is designed to bring out the power of the type combination- not every part of it. I would not consider this CaP a failure if it seldom used its poison STAB, so long as its typing was a major part of its success. Dry Skin negates Fire in realistic use, which strikes me as odd. Why do we hinder offensive Fire STAB, one of the reasons this CaP's typing can be good?
Storm Drain is a lesser issue, but it seems wierd that not only does CaP3 wall bulky waters, but they will fear being out if CaP3 gets Storm Drain. Why I view this as bad will be explained below.
I think that CaP3, by typing alone, has a good foundation to deal with bulky waters. The fact is, it has immunity to burn caused by Scald, and Toxic poisoning, as well as a STAB that can strike most back for neutral damage. That, and the bulky sets of the three bulky waters designated as "threatened by CAP3" all have uninvested Scald from base 80-85 special attack as their primary damaging option on their main set. If CaP3 can catch them outside of the rain, it can proceed to poison bulky Jellicent or Politoed, which is a big threat, in addition to taking what amounts to a weak, but super effective, attack. Alternatively, if it gets access to Sunny Day, it can put an even greater threat on the opposing bulky waters.
My proposal is for Cloud Nine or Air Lock. While it doesn't cause CaP3 to be immune to water, it does provide a very intersting niche in that it negates the rain boost on Scald, and shuts down Tentacruel's healing. From there, it can spread status. While Tentacruel would have to be dealt with by something other than ability, a single ability that covers all three of Politoed, Jellicent, and Tentacruel is likely to cover too much else. All this requires is some special bulk or speed to take or outspeed univested Scalds, and CaP3 can comfortably deal with or threaten the bulky waters. When an option like this exists, I don't see why we would have the need for a water immunity ability.
If neither of these two were deemed viable, Storm Drain and Water absorb do fit the concept, and I would be fine with them, even if I do view them as an easy way out.