Worth note: Chesnaught is a solid counter to Choice Band Locked and Belly Drum Azumarill
Chesnaught has base 64 Speed and Azumarill has base 50 Speed
Even with 124 Speed EVs, it brings Azumarill up to 167 speed, while Chesnaught with 16 speed EVs has 168
Assuming the EVs 252/240/16 Impish Chesnaught here are the following calcs
Calcs:
Defensively
252+ Atk Choice Band Huge Power Azumarill Aqua Jet vs. 252 HP / 240+ Def (custom): 38-45 (10 - 11.84%) -- 9HKO at best
252+ Atk Choice Band Huge Power Azumarill Waterfall vs. 252 HP / 240+ Def (custom): 75-89 (19.73 - 23.42%) -- possible 6HKO
+6 252+ Atk Mystic Water Huge Power Azumarill Aqua Jet vs. 252 HP / 240+ Def (custom): 120-142 (31.57 - 37.36%) -- 0.02% chance to 3HKO
Offensively
0 Atk (custom) Seed Bomb vs. 252 HP / 0 Def Azumarill: 218-260 (53.96 - 64.35%) -- guaranteed 2HKO
*Note that for Belly Drum Azumarill will already be at 50% HP
Chesnaught's Defenses vs Standard Mega Venusaur vs Standard Ferrothorn vs Standard Celebi:
Chesnaught (252 HP / 240 Def /16 Spe Impish)
+6 252+ Atk Mystic Water Huge Power Azumarill Aqua Jet vs. 252 HP / 240+ Def (custom): 120-142 (31.57 - 37.36%) -- 0.02% chance to 3HKO
Standard M. Venusaur (252 HP 252 SpA 4 Def Modest)
+6 252+ Atk Mystic Water Huge Power Azumarill Aqua Jet vs. 252 HP / 4 Def (custom): 178-210 (53.45 - 63.06%) -- guaranteed 2HKO
Standard Ferrothorn (252 HP 48 Def 208 SpDef Relaxed)
*Note that Ferrothorn is slower than Azumarill so Superpower or Ice Punch will be used
+6 252+ Atk Mystic Water Huge Power Azumarill Waterfall vs. 252 HP / 48+ Def Ferrothorn: 263-310 (74.71 - 88.06%) -- guaranteed 2HKO
+6 252+ Atk Huge Power Azumarill Superpower vs. 252 HP / 48+ Def Ferrothorn: 878-1034 (249.43 - 293.75%) -- guaranteed OHKO
+6 252+ Atk Huge Power Azumarill Ice Punch vs. 252 HP / 48+ Def Ferrothorn: 275-324 (78.12 - 92.04%) -- guaranteed 2HKO
Standard Celebi (252 HP 236 SpDef 20 Spe)
+6 252+ Atk Mystic Water Huge Power Azumarill Aqua Jet vs. 252 HP / 0 Def Celebi: 190-224 (47.02 - 55.44%) -- 17.58% chance to 2HKO
EDIT: Forgot to include Leftovers in calculations. FIXED
You forget Banded HugePower Stab Play Rough....