Goater, 35, was sent on by Southend boss Steve Tilson with United trailing 2-1 and the former Manchester City ace responded in style to head home an 89th minute equaliser.
It was no more than League One new boys deserved after a battling display but they still needed a stunning injury-time save by Daryl Flahavan from Jorge Leitao to earn a point.
Walsall, who had been chasing a club record eighth successive win, were made to pay for failing to convert their first-half chances when Wayne Gray gave Southend the lead in the 49th minute.
Matty Fryatt dragged Walsall back on level terms after 65 minutes before Leitao pounced with his second goal of the season three minutes later.
It looked good enough to rewrite the record books but Tilson's inspired decision to turn to Goater earned Southend the reward that they deserved.
Gray opened the scoring after Walsall midfielder Darren Wrack conceded a needless free-kick on the right wing. Carl Pettefer floated over a cross that the unmarked Gray headed home at the far post.
Wrack had the chance to make amends for his error less than 60 seconds later but drilled his shot into the side netting from just six yards.
Fortunately for Wrack, Fryatt proved to be more clinical in front of goal when he dragged Walsall back on level terms in the 65th minute.
Fryatt scored for the second time in two games when he stooped to head home a cross from Daniel Fox.
Walsall were revived by Fryatt's goal and Leitao completed their comeback after a neatly taken free-kick by Steve Staunton.
His through ball was turned across the face of goal by Wrack whose centre was stabbed home from close range by Leitao for what looked destined to be the winner.
But Goater was not to be denied and he rose high at the far post to head home a cross from skipper Kevin Maher before the heroics of Flahavan secured their point.