This is a hard choice. Both abilities would be phenominal on CAP3 and I wish we could do both. However after a lot of thinking, I would have to go with my original thought: for us to have CAP3 threaten water-types most effectively, we need to ensure that it takes the least amount of damage. Therefore I vote
Dry Skin
Because it would give CAP3 a reason to switch into water-types, which is something a Pokemon that threatens another is generally supposed to do. It still has a unique niche going for it, so it isn't a boring resist option like water absorb and storm drain would have been. I still agree with a lot of the arguments for supporting CAP3 to have drought, but I feel dry skin supports our aim better.