...or you know, can just run wisp as the fourth slot to help hinder the silly Azu switch ins and run something that sponges water attacks like no problem and ground like Gyara and or Rotom-W. It really isn't that difficult to give it some form of support I find.
will-o-wisp can only get you so far with water types and will not damper any specially based water types, which is generally what a lot of water types are. Also if you think will-o-wisp is your solution to azumarill, think again.
252+ SpA Sheer Force Camerupt Earth Power vs. 228 HP / 0 SpD Azumarill: 274-324 (68.8 - 81.4%) -- guaranteed 2HKO after burn damage
(274, 277, 280, 285, 288, 291, 294, 297, 300, 304, 307, 310, 313, 316, 319, 324
252+ Atk Choice Band Huge Power burned Azumarill Aqua Jet vs. 252 HP / 0 Def Camerupt: 240-284 (69.7 - 82.5%) -- guaranteed 2HKO
(240, 242, 246, 248, 252, 254, 258, 260, 264, 266, 270, 272, 276, 278, 282, 284
252+ Atk Choice Band Huge Power burned Azumarill Waterfall vs. 252 HP / 0 Def Camerupt: 476-564 (138.3 - 163.9%) -- guaranteed OHKO
(476, 482, 488, 494, 500, 506, 512, 516, 522, 528, 534, 540, 546, 552, 558, 564)
4 Atk Camerupt Earthquake vs. 16 HP / 0 Def Azumarill: 153-180 (44.3 - 52.1%) -- guaranteed 2HKO after burn damage
(153, 154, 156, 157, 159, 162, 163, 165, 166, 168, 171, 172, 174, 175, 177, 180
252+ Atk Huge Power burned Azumarill Aqua Jet vs. 252 HP / 0 Def Camerupt: 162-192 (47 - 55.8%) -- 74.2% chance to 2HKO
(162, 164, 164, 168, 168, 170, 174, 174, 176, 180, 180, 182, 186, 186, 188, 192)
252+ Atk Huge Power burned Azumarill Waterfall vs. 252 HP / 0 Def Camerupt: 320-378 (93 - 109.8%) -- 56.3% chance to OHKO
(320, 324, 326, 330, 336, 338, 342, 344, 350, 354, 356, 360, 366, 368, 372, 378)
In order for camerupt to deal with azumarill it has to be an assualt vest set, already burned, and there would still only be a 43.7% chance of surviving a waterfall. So will-o-wisp would still not be enough to stop azumarill. Also you could support any pokemon with having pokemon to sponge up their weaknessess but that doesn't automatically make it a good pokemon. Granted I'm not trying to shit on mega camerupt because it is still a good mega for sure. But would you have mega camerupt take up your mega slot and not mega zard Y, not remotely! And I hate charizard too. But hey what do I know?