Water/Ice is an incredibly potent combination, especially paired with Freeze-Dry.
However in the interest of variety, some other types that could compliment Water offensively are...
Poison: While there are better STAB options on the special side of Poison, both are usable. Namely, this allows AT Snaelstrom to threaten Cresceidon and Ogerpon-Wellspring, two big potential checks. The common chance for Poison status also play into our seeming hit-and-run play style. There's also the very Download-friendly Shell Side-Arm
Psychic: Like Poison, there's a slight bias to the special side, but both are usable. To goal here is instead to target two other potential checks in Arghonaut and Mollux. There's also the very niche application of Download for Future Sight.
However in the interest of variety, some other types that could compliment Water offensively are...
Poison: While there are better STAB options on the special side of Poison, both are usable. Namely, this allows AT Snaelstrom to threaten Cresceidon and Ogerpon-Wellspring, two big potential checks. The common chance for Poison status also play into our seeming hit-and-run play style. There's also the very Download-friendly Shell Side-Arm
Psychic: Like Poison, there's a slight bias to the special side, but both are usable. To goal here is instead to target two other potential checks in Arghonaut and Mollux. There's also the very niche application of Download for Future Sight.