Just as Watford looked to be heading for a crucial victory, an Alireza Jahanbakhsh cross was turned into the roof of his own net by Mariappa to level the tie with just over 10 minutes remaining.
The Hornets had suffered successive defeats against Aston Villa and Everton in the Premier League before the trip to the south coast, and Pearson’s side remain in the bottom three despite claiming a point at the Amex.
The Seagulls remain without a win in the Premier League this year, but have moved one point further away from the drop zone.
The home side were caught out in the 19th minute and Watford made the most of a misplaced pass to take the lead with their first shot of the match.
The ball fell to Doucoure who drove his way through the Seagulls defence before firing a powerful effort past Ryan in the Brighton goal.
Brighton struggled to get going in the midfield after going behind, and had difficulties creating chances in the later stages of the first half.
Brighton came close to levelling the match at the start of the second half, but Ezequiel Schelotto’s ball across the box was just ahead of the sliding Glenn Murray at the far post.
The home side continued to look for the equaliser, but struggled to get the ball past the resolute Watford defence.
Aaron Mooy nearly levelled the tie in the 77th minute for the home side, but Foster made a crucial save to deny the Australian his third goal of the season with an outstretched leg.
Just moments later, the home side found the equaliser, when substitute Jahanbakhsh’s cross was tapped past Ryan and into his own net by Mariappa, who was under no pressure at the far post.
Neither side were able to claim a winner in the final minutes, as the points were shared.