Rovers had to score in the dying seconds to earn a point after throwing away a half time lead.
Coppinger had Rovers first effort on goal but Mariappa blocked his shot in the Watford defence.
Rovers had enjoyed plenty of the early pressure but Watford had the next chance when former loan player Jordan Mutch fired wide of the target.
Martin Taylor then headed over from a Watford corner as the home side came into the game.
Billy Sharp had a half chance but his shot went wide of the target for Rovers.
Martin Woods was next to have a go but his shot sailed over the crossbar.
Rovers took the lead on thirty-three minutes when a quick break saw Billy Sharp and John Oster exchange passes and Simon Gillett was there at the end of it to fire past Loach in the Watford goal.
Watford were not troubling Neil Sullivan in the Rovers goal, Mutch was next to have an effort for the home side but once again he shot wide as the game neared half time.
Neil Sullivan made his first save of the afternoon when he denied Danny Graham.
James O`Connor had Rovers first real effort of the second half but saw his shot go wide of the post.
Watford levelled half way through the second half when substitute Marvin Sordell headed in from close range after a ball into the box by Cowie.
The game was turned on it`s head within seven minutes when Sordell scored his second goal when his low shot beat Sullivan from the edge of the area.
Rovers made a triple substitution with twelve minutes remaining as they searched for an equaliser.
Rovers equalised in the closing moments when defender George Friend poked in from close range.
It was to prove to be the last chance of the game as Rovers managed to earn a point at Watford for the third season in a row.
Loach, Hodson, M. Taylor, Mariappa, A. Taylor, Eustace, Cowie, McGinn (Sordell 55), Mutch (Walker 90), Graham, Buckley (Jenkins 88)
Bennett, Bryan, Deeney, Gilmartin,
Sullivan, O`Connor, Martis, Lockwood (Dumbuya 79), Friend, Stock (Mason 79), Oster (Shiels 79), Coppinger, Sharp, M. Woods, Gillett
G. Woods, Fairhurst, Hird, Wilson
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