Semedo spurned another great opportunity to put Wolves ahead in the 62nd minute after a deft touch from Raúl Jiménez beat the Watford defence, but the Portuguese missed the target with his tame effort.
The away side again went close seven minutes later as Jiménez headed a Moutinho free-kick just wide, and skipper Conor Coady then looped a header over from a corner.
Back at the other end, Danny Rose’s tricky run put Wolves on the back foot, and Hernández then kept them on red alert with a teasing cross into the six-yard area.
But the visitors took the lead in the 74th minute, Sierralta turning a header into his own net from a Wolves corner under pressure from the opposition attack.
And as the Hornets pushed for a leveller late on, away replacement Hwang doubled the deficit, with a close-range second sealing the points after Bachmann had initially made a smart save from point-blank range.