I don't think I should pick Araquanid over Galvantula, as it is a water-type and I already have Primarina. Plus, Galvantula's electric typing is very useful against flying and water types and Life Orb helps him one-shot some pokemon that don't have weakness.
As for Primarina, I did have Hydro Pump on her at first, but after missing it like 50% of the time (I have the worst of luck), I returned to a more stable move. Choice specs would give her a nice power boost, but I really hate the fact that I am locked into a move and have to switch out to do anything.
Sometimes type overlap is necessary to make a team work. For example, Kartana and Tapu Bulu are used together despite being Grass types because the latter can summon Grassy Terrain with Grassy Surge to power up its and Kartana's Grass STABs.
If you still don't like Araquanid and Primarina being used together, I'd use Shuckle as a Sticky Web setter as it can also set up Stealth Rock. Galvantula isn't that great of a Sticky Web setter compared to Araquanid or Shuckle; because of its frailty, it can generally only get up Sticky Web once, then get KOed shortly after:
-Sticky Web
-Stealth Rock
-Encore
-Infestation/Rock Tomb/Toxic
Nature: Relaxed
EVs: 252 HP/168 Def/88 SDef
Item: Mental Herb
Ability: Sturdy
While it's true that Primarium Z gives Primarina a one-time nuke, Sparkling Aria isn't that great of a move on its own. Hydro Pump packs more power, while Scald has a decent chance to burn its mark. Specs Primarina is a decent wallbreaker because of its high Special Attack and little penalty of being locked into its STABs. Even though Toxapex walls Primarina's STABs, it still has Psychic to hit it hard.
If you want to have a viable Electric Pokemon to use, Tapu Koko is the prime pick as it's faster and can summon Electric Terrain to make its Electric attacks hit harder:
-Thunderbolt
-Dazzling Gleam
-Volt Switch
-Hidden Power (Fire/Ice)
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Choice Specs
or
-Thunderbolt
-Dazzling Gleam/U-turn
-Hidden Power (Fire/Ice)
-Roost/Calm Mind
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Electrium Z/Fairium Z
or
-Wild Charge
-U-turn
-Hidden Power (Ice)
-Taunt
Nature: Naive
EVs: 252 Atk/4 SAtk/252 Spe
Item: Electrium Z
or
-Thunderbolt
-Hidden Power (Ice)
-U-turn
-Taunt/Volt Switch
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Magnet/Zap Plate