I personally wouldn't use it as something you would build a rain team around. It's really slow, is hard countered by the relatively common mega salamence, and is scared out by koko, ruining your momentum. If you were to try out rain, I'd try Kingdra before anything else, never gets outsped and its Water/Dragon coverage is, at least before bank comes out, unresisted barring the somewhat rare (and easy to cover!) Azumarill, and the more common Bulu and Fini, one of whom is taken care of by ice beam, the other of whom needs another teammate.
If you were to end up using Araquanid anyway, I'd advise that you use its numerous positives to help out your team. It resists Mamoswine's STAB's and thus comes in on it with impunity, as well as anything choice-locked into earthquake. It's capable of wallbreaking virtually anything non-resistant to water if you have a choice band (Celesteela, the thing you'd actually want to 2hko with cb, sadly averts a Liquidation 2HKO without Band if it's phys def).
Araquanid @ Choice Band
Ability: Water Bubble
Level: 50
EVs: 92 HP / 252 Atk / 4 Def / 4 SpD / 156 Spe
Adamant Nature
- Liquidation
- Lunge
- Crunch
- Leech Life
This isn't LITERALLY Liquidation/filler/filler/filler but... Water Bubble makes Liquidation a bad play in only the cases of:
Liquidation is not very effective and Lunge/Leech Life are super effective (Mega Gyarados is a big mon in this category, but I wouldn't stay in on it if it hasn't mega'd since it can 50/50 you out)
Liquidation is 4x resisted or has an immunity.
Literally, i calculated it vs regular salamence with an intimidate down and Liquidation LITERALLY DOES MORE THAN ICE BEAM (-1 252+ Atk Choice Band Water Bubble Araquanid Liquidation vs. 4 HP / 0 Def Salamence: 96-114 (56.1 - 66.6%) -- guaranteed 2HKO) vs (0- SpA Araquanid Ice Beam vs. 4 HP / 0 SpD Salamence: 88-104 (51.4 - 60.8%) -- guaranteed 2HKO)
as well as the obvious place where you are forced to switch out. EV's are to creep Alolawaks who outspeed 0 speed p2, which also conveniently outspeeds 0 speed celesteela. I don't have tons of experience with it, so feel free to correct me if you see something that may be better!
Teammates it loves are things that dislike celesteela and mamoswine, two mons that immediately come to mind are garchomp and mega salamence. Additionally, mons that dislike earthquake like its help, such as Mega Metagross, Alola-Muk, and Aegislash appreciate Araquanid's presence. It appreciates things that can take its incredibly common weaknesses, Celesteela and Porygon2 are two things that are capable of going toe-to-toe with Mega Salamence (p2 can only take two double-edges with a physically defensive set, 196 Defense Bold if you're curious) and things that can defeat Tapu Koko, something that basically gains free momentum against it (for example, Mamoswine!)
Good luck building!