Ismaël rang the changes with little under 25 minutes left, as Asprilla, Jamal Lewis and Jake Livermore made way for Martins, Ince and Giorgi Chakvetadze, and the latter quickly got the fans up off their seats with some tricky footwork.
Hamer had to stay alert to tip a lofted Cafú attempt past his post, before a Cohen Bramall strike - and then an Ayala chance at the back post - reminded the Hornets of the quality possessed by their opponents.
Rhys Healey came into the fray with six minutes to go, while Rotherham’s Cafú went off without being replaced due to the South Yorkshiremen having already made five replacements, and Ince quickly made the numerical advantage count with a deft finish to make it 4-0.
Kayembe could have had a second, when he audaciously flicked the ball narrowly wide past Johansson and the woodwork, and sub Tom Dele-Bashiru then made the Swedish stopper work with a fierce drive from 18 yards.
Watford continued to push forward deep into seven minutes of added time, and Martins wrapped up the convincing win with a fifth, slotting into the corner after Healey had initially been set through on goal by Ince.