An early Troy Deeney penalty sent the Hornets ahead on home soil, but Ayew's strike in the 73rd minute saw 10-man West Ham claim the point they needed to move into the top half of the Premier League table.
Watford made two changes to the team that started their 2-0 defeat against Manchester United last time out, with Daryl Janmaat and Valon Behrami taking the places of Craig Cathcart and Sebastian Prodl.
West Ham, meanwhile, were unchanged from their 2-2 draw with West Brom last time out. It had been thought that Andy Carroll could make the bench despite a hamstring problem, but the striker was left out altogether. As a result, Antonio operated through the middle with Manuel Lanzini and Robert Snodgrass supporting from wide positions.
Watford were awarded a penalty in just the second minute of the clash when Cheikhou Kouyate clumsily brought Mauro Zarate to the deck, and Deeney stepped up to send the Hornets into a third-minute lead on home soil before West Ham really had the chance to take in their surroundings.
On-loan AC Milan attacker M'Baye Niang almost doubled Watford's lead in the sixth minute when he struck a thumping effort from distance, but the ball just cleared Darren Randolph's crossbar as the Hammers just looked to survive in the early moments.
West Ham's first half-chance of the match arrived in the 10th minute when Antonio delivered a low cross towards the near post, but Miguel Britos just about managed to turn the ball away from his own goal, before Etienne Capoue had a strike deflected just wide of the post down the other end.
The visitors had another opportunity in the 18th minute when Sofiane Feghouli robbed Jose Holebas of the ball before setting up Snodgrass inside the Watford box, but Younes Kaboul was on hand to make a stunning defensive block.
Snodgrass again tried his luck in the 31st minute as West Ham continued to enjoy possession inside the Watford half, but the former Hull City midfielder saw his effort fly high and wide of Heurelho Gomes's goal.
Lanzini should have done better from an advanced free kick in the 36th minute as Watford dropped deeper and deeper approaching the end of the first period, before Snodgrass passed up a brilliant chance in first-half stoppage time after Gomes had palmed an Aaron Cresswell cross straight into the path of the Scotland international.
The first half ended on a sour note for Watford, however, as Argentine attacker Zarate was stretchered off the field after picking up what looked to be a serious knee injury, putting the rest of his season in severe doubt.
Abdoulaye Doucoure was Zarate's replacement late in the first period, but neither manager made a change at the interval, which meant that Antonio continued through the middle for West Ham despite his lack of support in the opening 45 minutes.
The visitors had a shout for a penalty in the 50th minute when Kouyate went down in the box under pressure from Niang, but the referee was not interested and instead awarded a free kick to the hosts, which brought a furious reaction from Slaven Bilic.
West Ham had a super chance to level just past the hour when Antonio nodded the ball towards Jose Fonte, who was lurking at the far post, but the January signing from Southampton could not place his header past the diving Gomes.
Ayew was introduced in the 65th minute as Bilic looked to turn his team's dominance into a goal, but Watford, marshalled by Kaboul, continued to stand strong approaching the final 20 minutes of action.
Antonio had an outstanding chance to put West Ham level in the 69th minute after meeting a low cross from Cresswell, but the 26-year-old somehow nodded wide of Gomes's post as Watford rode their luck once again.
The equaliser was always coming, however, and the Hammers finally levelled the scores in the 73rd minute when Ayew tapped home from close range after Antonio's low strike had hit both posts and dropped into the Watford box.
West Ham were forced to play the final five minutes with 10 men after Antonio picked up a second yellow card for handball, and the visitors came close to conceding a second late on when Britos rose inside the Hammers' box to meet a corner, but the centre-back's header was just wide of the post.
Watford came even closer to a winner in second-half stoppage time when Deeney placed a header towards Isaac Success after West Ham failed to clear a corner, but the substitute somehow bundled the ball wide of the post.
Next up for Watford is a home game against Southampton on March 4, while West Ham will attempt to return to winning ways when they welcome league leaders Chelsea to the London Stadium on March 6.