But hopes of Woking, 110 places adrift of the Hornets, enjoying a shock win were dented by Will Hughes’s 13th-minute curled shot from the edge of the area.
Watford doubled their lead 16 minutes from time through Troy Deeney.
Millwall came from a goal down to beat Championship rivals Hull 2-1, substitute Shane Ferguson scoring twice inside the closing eight minutes, with QPR also winning 2-1 against second-tier leaders Leeds.
On Saturday, holders Chelsea cruised to a 2-0 win over Nottingham Forest in what seems certain to have been Cesc Fabregas’s final match for the Blues.
It was the same result as Manchester United beat Reading to make it five wins out of five under caretaker boss Ole Gunnar Solskjaer.
Arsenal, the record 13-times FA Cup winners, won 3-0 at Blackpool, West Ham beat Birmingham 2-0 and Everton defeated Lincoln 2-1.
Saturday also saw Championship Bristol City beat top-flight basement club Huddersfield 1-0.
Elsewhere, in an all-Premier League clash, Brighton won 3-1 away to south coast rivals Bournemouth.
Second division Derby, managed by ex-England midfielder Frank Lampard, came from 2-0 down to draw 2-2 with Premier League strugglers Southampton, with Newcastle held 1-1 by second-tier Blackburn.
Liverpool will complete a third round that started with Tottenham’s 7-0 thrashing of Tranmere on Friday when they face top-flight rivals Wolves on Monday.