Jones again had a chance to put the hosts in front as his 40th-minute header from Apter’s floated free-kick was again well saved by Selvik.
Watford improved after the break as Mamadou Doumbia’s early cross flashed across the six-yard box, but the Hornets were lucky to not concede a penalty 10 minutes later when Jeremy Ngakia appeared to commit a foul in his own box.
Charlton continued to create chances as Charlie Kelman had a close-range effort saved while Apter set Sonny Carey off down the right flank but his near-post effort was stopped by Selvik.
Watford’s best effort came midway through the half when Hector Kyprianou headed over an Ngakia cross as the Hornets struggled to find a cutting edge up front having started with Danish summer signing Luca Kjerrumgaard.
The impressive Apter curled a left-footed effort wide in the final few minutes before Watford had a penalty claim turned down in stoppage time when substitute Nestory Irankunda felt he was fouled by debutant goalkeeper Thomas Kaminski.
But Charlton got the goal their play deserved late on: Conor Coventry’s corner from the right was headed back to Knibbs by Jones and the former Reading man – who scored 16 goals last season – celebrated his debut as he side-footed home from six yards.
More to follow.