Question

Does the ability Water Bubble double the power of Hydro Vortex (the Water Z-Move)?

Answer 1

Yes.
source

Comments

Thanks! For clarification, I assume the non-Shiny Araquanid had Water Absorb?

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No, they were both shiny. It's just that Showdown! doesn't have the Araquanid shiny back sprite, so the one that happened to be facing away from you was shiny.