Watford led 2-0 at half-time courtesy of goals from Fernando Forestieri and Troy Deeney, but City rallied after the break as Aguero's treble was complimented by Aleksandar Kolarov's fortuitous strike to spare Manuel Pellegrini's side's blushes.
Giuseppe Sannino's men were rewarded for a fine opening period with two goals. First the impressive Forestieri showed a good turn of pace before finishing neatly from the left, before Troy Deeney capped a swift Watford breakaway.
After a lacklustre opening, City showed a marked improvement after the break and halved the deficit through Aguero's tap-in before he curled home a second with his left foot.
The hosts uncharacteristically struggled to maintain possession in the early stages, but fashioned a good opportunity after 11 minutes when Kolarov flicked Jesus Navas' corner invitingly across goal and just beyond the reach of Joleon Lescott.
Watford dealt well with City's attacking force and deservedly went in front after 21 minutes, when Forestieri outpaced Martin Demichelis before calmly slotting under Costel Pantilimon in the City goal.
A slick move on the stroke of half-time created the opportunity for a third goal, but Daniel Pudil's fierce effort found the side-netting.
The deficit was reduced after 58 minutes, when Kolarov's teasing cross was met by a sliding Dzeko and, though his effort was saved, Aguero was on hand to convert the rebound.
Watford's resistance finally broke in the 78th minute. Navas drove forward and picked out Dzeko, who in turn laid into the path of Aguero and the striker curled the ball into the top corner from the right-hand side.
The comeback was complete in the 85th minute when Kolarov's hopeful 30-yard strike squirmed under the despairing Bond.
