A rare Ashley Williams goal, headed home from inside the six-yard box after 27 minutes, in 18 league attempts and lift them out of the bottom three.
It was far from pretty at times but that will not worry a Swansea side who inflicted a fourth straight league defeat on a Watford side suffering their worst run of the season.
“This is the situation we find ourselves in. It’s not about pretty football: we’ve got to fight,” said a relieved Williams last night.
Guidolin’s appointment was confirmed before kick-off and the former Parma, Palermo and Udinese coach took his place in the stand as Swansea sought to escape the bottom three.
Guidolin witnessed a pretty drab opening with Jack Cork’s wayward effort the only shot in the first 20 minutes.
Watford might have been awarded a penalty after 21 minutes when Kyle Naughton handled right on the edge of the area. Watford appeals were conspicuous by their absence, however, and the Hornets were soon behind.
Ki Sung-yueng retrieved a loose ball down the right and his cross was met by Williams, who took advantage of Miguel Britos misjudging the flight by directing his header goalwards for the first goal from any Swansea defender since March 2014.
Fabianski, Naughton, Fernandez,Williams, Taylor, Britton, Cork, Ki (Amat 87), Sigurdsson, Ayew (Gomis 82), Routledge (Barrow 66). Not Used: Emnes, Nordfeldt,Montero, Rangel.
Gomes, Nyom (Paredes 74), Cathcart, Britos, Ake, Capoue, Behrami (Oulare 87), Watson, Jurado ,Ighalo, Deeney.
Prodl, Guedioura, Anya, Abdi, Arlauskis.