A combination of injury and the Africa Cup of Nations has stunted Ayew's first season for the Hammers, having joined for a club-record fee from Swansea, but his strike from the bench secured a deserved point for the visitors.
Watford had looked set to climb above West Ham and into the top half of the table as they led for 70 minutes, but injuries to Zarate and Daryl Janmaat either side of half-time took their toll.
West Ham surrendered a two-goal lead to lose the reverse fixture at home in September, as one of their first league matches at London Stadium hinted at the topsy-turvy nature of their season. And, with both sides seemingly safe from the drop, another entertaining affair appeared to be on the cards when Troy Deeney converted an early penalty. But chances were few and far between for the remainder of the first half, and Ayew's goal would see the sides share the spoils, with a late red card for Michail Antonio denying Slaven Bilic's men the opportunity to chase a winner.
While West Ham started without a recognised forward, Walter Mazzarri was able to name a front three – two of whom were involved as Watford took the lead within three minutes. Ex-Hammer Zarate ran onto Deeney's pass inside the area on the left, before going to ground under Cheikhou Kouyate's clumsy challenge and earning a penalty. Deeney rattled the spot-kick into the bottom-left corner and Watford were soon on the attack again, seeking a second, as M'Baye Niang lashed over the crossbar.
But the hosts' momentum slowed as the game went on, with Younes Kaboul blocking from both Robert Snodgrass and Antonio at the other end, while West Ham could not profit as Heurelho Gomes pushed a low cross into the centre of the box. Watford then lost Zarate to injury moments before the break, having earlier gone down with a knock to his leg. After receiving lengthy treatment on the pitch, the 29-year-old was stretchered off to applause from both sets of supporters.
45+9: #watfordfc's Zarate finally stretchered from the field of play to applause from both sets of fans.
— Watford FC (@WatfordFC) February 25, 2017
Janmaat went down shortly after the restart and was also substituted, with pressure growing on the Watford goal as Kouyate and Aaron Cresswell saw respective penalty appeals waved away.
Opportunities finally began to arrive for the visitors, but Gomes crucially dived on Manuel Lanzini's teasing centre and then sprang to his left to keep out Jose Fonte's header. Although Antonio nodded narrowly wide after storming onto Cresswell's whipped delivery, he was heavily involved in the equaliser. Sprinting clear of Kaboul, the former Nottingham Forest winger's finish clipped both posts before falling kindly at the feet of Ayew, who could hardly miss.
West Ham kept pushing for a decisive second, but when Antonio handled to prevent Isaac Success from launching a counter, he received a second yellow and reduced the visitors to 10 men. Watford nearly clinched the points in a hectic goalmouth scramble, but Success could not turn the ball past Darren Randolph in the game's last action.
Key Opta stats:
– Troy Deeney has now scored in four of his last five Premier League appearances for Watford.
– The striker has scored nine of his 21 Premier League goals from penalties (43 per cent) – only Harry Kane has scored as many penalties in the competition since the start of last season.
– Watford have kept just one clean sheet in their last 16 Premier League matches.
– Jose Holebas picked up his 12th yellow card of the season in this match – more than any other Premier League player in 2016-17.
– Michail Antonio picked up the first red card of his professional career.
– West Ham United have now lost just one of their last seven Premier League away games (W3 D3 L1).
– Andre Ayew's goal was the 100th scored in the Premier League for West Ham under Slaven Bilic.