Araquanid is a bit of a weird pokémon, but very strong. Although at first glance that base 70 attack stat seems pretty awful, its signature ability Water Bubble compensates greatly, doubling the power of all its water moves, giving it a burn immunity, as well as a fire resistance. Araquanid can hit very hard with Liquidation. When fully invested with a Brave nature, it has the equivalent of a base 192 attack stat, higher than behemoths like Mega Heracross, Mega Mewtwo X, and Mega Rayquaza, only beaten by Mega Medicham (which can't hold items unlike Araquanid). However, this boost only applies to its water attacks, leaving offensive sets helpless against bulky foes that water doesn't work well against like Vaporeon and Water Absorb Mantine, or even Dry Skin Toxicroak.
Therefore, it might be interesting to look at defensive supporting sets too. Araquanid does possess good bulk, especially on the special side. Three weaknesses and six resistances (including fire) isn't bad at all, it can patch up its physical bulk with Lunge, Iron Defence, or Z-Aqua Ring, and has access to recovery with Aqua Ring and Leech Life. So what kind of support options does it have? It can lay down Sticky Web, it can support with Wide Guard, it has some other tricks like Power Split, Wonder Room, and Magic Room, but the most unique might be Entrainment for doubles. Pick your partner carefully, but there are several pokémon that love to get Water Bubble.
Gyarados is an interesting partner. Not only does it Intimidate enemies to patch up Araquanid's weaker defensive side, once it has triggered Intimidate that ability is worthless for the remainder Gyarados is out anyway, so there's no harm to replace it with Water Bubble. Gyarados is already quite strong, but with a double-power Waterfall potentially boosted by Dragon Dance it suddenly becomes terrifying. In unrestricted doubles like the free battle spot regular Kyogre is also an interesting partner. After it sets up rain Drizzle doesn't do anything anymore, so replace it with Water Bubble and then destroy everything with base 220 Origin Pulse + rain (run Kyogre with Waterium Z for extra fun, Water Bubble Hydro Vortex in the rain one-shots Eviolite Chansey and has a good chance to OHKO Lugia through Multiscale before it can Whirlwind you out). In restricted formats like VGC or rated battle spot other rain setters like Politoed are also interesting options.
Power Split is worth mentioning, you can do gimmicks with it, reducing enemy offensive power while increasing your own, but it's unreliable and doesn't really give a very large boost in a lot of cases. For example, if you combine it with Brave Aegislash in a doubles format and hit your partner right after Aegislash attacks the enemy (so getting Aegislash in its blade form), Araquanid's attack stat goes up from 134 to 178, a boost of 32%, it's effectively weaker than Howl or Meditate. Aegislash can nullify its own stat drop just fine, it gets negated as soon as Aegislash changes form, but it's not really all that worth it.
