The Lilywhites lead at half-time thanks to goals from Sean St Ledger and Leon Clarke, but the Hornets fought back to earn a draw through Matt Whichelow and Adam Thompson.
Sheffield United remain in the third relegation spot after a 1-1 home draw with Reading.
Shane Long looked to have the Royals on course for the win, but Daniel Bogdanovic converted a penalty with two minutes of normal time to play.
Ipswich Town put on the display of the evening, as they went to Doncaster Rovers and recorded an impressive 6-0 success.
Young forward Connor Wickham scored a hat-trick for the Tractor Boys, with Sam Hird, Colin Healy and Gareth McAuley also netting.
Cardiff City were held to a 1-1 stalemate at home to Burnley. They took the lead through Michael Chopra, with Steven Thompson levelling for the Clarets.