What it does
Water Bubble has two effects: first, all Water-type moves used by the Pokémon deal ×2 damage, applied before type effectiveness is calculated; second, the user is completely immune to Burn. The ×2 Water damage is the defining mechanic. Combined with STAB (×1.5), Water-type moves under Water Bubble deal three times the unmodified base power worth of damage, making Araquanid's printed 50 base Special Attack deeply misleading. For Water-type moves, it effectively operates as though it had 100 base Special Attack.
Who runs it
Araquanid is the only relevant carrier at 99% usage. Water/Bug typing gives it Water STAB. Its physical Attack sits at 70, and its Special Attack at 50, but Water Bubble makes both of those numbers irrelevant for Water-type moves. Liquidation is the physical Water option; Scald or Surf cover the special side. Burn immunity is also a meaningful bonus — facing Flame Body users or opposing Will-O-Wisp doesn't threaten to cut Water-type output through the halved physical damage that Burn imposes.
How to use it
In doubles, Araquanid is a hard counter to Fire-type-heavy teams. The ×2 damage multiplier on Water moves, on top of super-effective type matchups against Fire, compounds into very high damage. Surf hits both opponents as a spread move, though it also hits your own partner, so team positioning matters. Single-target Water moves avoid that issue. Araquanid's below-average Speed means it usually moves after faster threats, so Trick Room support or speed control helps it land hits before being targeted.
Tips & strategy
The ×2 multiplier applies to both physical and special Water moves, so Liquidation benefits just as much as Scald or Surf. For physical Water moves, 70 base Attack × Water Bubble effectively acts like 140 — considerably stronger than the stat suggests. The biggest threats to Araquanid are Flying-type moves (2× weakness). Electric moves are 2× (not 4×), and Grass moves are neutral (1×) due to Bug's resistance to Grass cancelling Water's weakness. Scout for Flying coverage on the opponent's team before committing Araquanid to the field. The Water Bubble burn immunity also specifically counters Flame Body users — contact moves against Araquanid won't trigger burn, which is a practical benefit when using Liquidation into bulky Fire types.