Boro went ahead with a well worked Albert Adomah goal in the first half but Watford levelled through Troy Deeney before the break.
The visitors took the lead in the second half through Fernando Forestieri but Ayala squared things up in the third minute of added time.
Watford went close in the 11th minute as a slotted pass from Lewis McGugan sent Marco Faraoni racing free into the box but Steele did well to palm away his angled 12-yard drive.
Boro went close on 19 minutes as Kei Kamara's downward header from an Adomah cross was blocked.
But Boro took the lead with a well worked opener five minutes later.
Marvin Emnes beat two men then teed up Kamara, who slotted diagonally into the box to pick out the run of Rhys Williams - who backheeled for Adomah to rifle inside the near post.
Watford levelled out of the blue on 32 minutes as Marco Cassetti played a quick chip over a static Boro defensive for Deeney to nip through unchallenged and slam home.
Nine minutes before the break Boro won a free-kick just outside the box but Grant Leadbitter's stinging shot was fisted away by goalkeeper Manuel Almunia.
Watford almost broke through in the 40th minute as McGugan sent Daniel Pudil into the box - but he screwed his shot wide.
On the stroke of half-time Boro went close as Emnes crossed for Kamara and his scissor-kick sent keeper Almunia scrambling across to save at far post.
Watford's first foray forward of the second half came in the 57th minute as Murray broke down the right and with no support arriving and Ben Gibson closing in fast he opted to shoot and his low effort drifted well wide of the far post.
Boro had a strong shout for a penalty just past the hour as Gibson put in a glancing header that appeared to hit defender Gabrielle Angella on his outstretched arm but referee Paul Tierney waved play on.
Up the other end Watford threatened as Deeney held off Seb Hines outside the box but he poked his shot wide.
Watford grabbed the lead after a defensive blunder on 73 minutes.
Deeney played a ball forward for Forestieri and defender Ayala missed a chance to clear, leaving the striker free to get into the box and slam a low shot that Steele got a hand too but could not keep out.
Watford threatened with 10 minutes remaining as Murray cut inside as the Boro defenders dithered and stood off him - but he shot just over.
Boro were almost back in it on 86 minutes when Ayala flicked on a Jacob Butterfield ball - but Kamara headed straight at the keeper from close range.
However, the home side salvaged a point deep into stoppage time as Ayala headed in from Leadbitter's corner.