I think it can be good:
252+ Atk Choice Band Araquanid Leech Life vs. 0 HP / 0 Def Latios: 344-408 (114.2 - 135.5%) -- guaranteed OHKO
252+ Atk Choice Band Araquanid Leech Life vs. 252 HP / 252 Def Latias: 242-288 (66.4 - 79.1%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Choice Band Araquanid Leech Life vs. 252 HP / 0 Def Tangrowth: 236-282 (58.4 - 69.8%) -- guaranteed 2HKO
If it goes special, it still has the same problem with non Water type moves + less BST on the special side.
Nice calcs. That does look better than I expected. Still, my experience in the previous metagame was that physical attackers have to hit really hard, or carry Swords Dance to be even a little effective because everyone carried either Defensive Suicune, Defensive Cresselia, Defensive Porygon 2 or Defensive Ferrothorn. It would get walled pretty hard by all of those, even Cresselia (missing the 2 hit KO) despite being super effective..
And then there is the disadvantage of being locked into a not that strong bug type move. Heatran comes in (example) sets up a sub, gets annoying with Toxic.. I think there are lots of Pokemon able to take advantage of that. Fun fact is that Liquidation hits harder than Leech Life if it's not very effective vs neutral, or neutral vs super effective..
The last problem I see is that it's sooo slow and it doesn't outright KO threats like Garchomp. It will normally have to take a hit and it's Defense/HP are lacking. A strong stone edge means goodbye and so does Stealth Rock more or less so switching in and out can also be problematic. Trick Room will not change it's problems mostly because the mentioned walls remain, and it's tricky (haha) to set up reliably.
I think I would go like this;
Specially defensive; 252 HP/252 Sdef @ Leftovers
Leech life/protect (for extra recovery)
Scald/Liquidation (depending on what the team needs; power vs burns)
Substitute
Toxic/Mirror Coat (Mirror Coat can be a nasty suprise for Electric types, but my guess is it'll get mostly Rock Slided/Stone Edged)
Idea would be to come in on a special attacker (almost anyone), set a sub on the switch and either toxic or scald/liquidation the incoming mon. Then either switch or use Leech Life/Protect for recovery depending on what came in. Tapu Fini will be a massive counter to this, but yeah..