Z-Freeze-Dry does NOT deal super effective damage against Water-types.
In this test replay, I revealed mono Freeze-Dry (meaning Freeze Dry is the only damaging move) on the Vanilluxe set to make sure you don't think I'm using another Ice move. I Subzero Slammer with Freeze-Dry as the Ice move against Alomomola, and it ended up being a resisted hit instead of a super effective hit that Freeze-Dry would otherwise do. This interaction is the same in-game, as if you use Subzero Slammer with Freeze-Dry as the Ice move against a Water-type, the Z move won't deal super effective damage to it. The Z-Freeze-Dry would either be not very effective or be neutral.
TL;DR Z-Freeze-Dry doesn't deal super effective damage to Water-types, as the Water-type will resist it or be neutral to it. Basically, don't use Freeze-Dry as the base move for Subzero Slammer, as it will have a weaker power than if you were to use Ice Beam or Blizzard for the base move for Subzero Slammer.
