Summers said the club has "had a tough few years" and the decision to sack Watters was not an easy decision but the result of a considered process.
Summers said the rationale for the sacking would remain confidential between the Board and Watters.
But he rejected any suggestion a personality clash between the club's head of football Chris Pelchen and Watters was behind the axing, saying it was "not a factor".
There is speculation that former Port Adelaide premiership coach Mark Williams, head of development at Richmond, is a strong contender to replace Watters.
Summers said the club had a list of potential new head coaches to sift through but insisted none had yet been approached.
Watters inherited an ageing list from Ross Lyon, who made a shock move to Fremantle after the 2011 season.
Watters guided the Saints to ninth in his first season last year.
But this year, they came 16th with just five wins.
They have since lost star player Nick Dal Santo, who switched to North Melbourne as a free agent.
That came a year after they lost another star Brendon Goddard to Essendon as a free agent.
Watters had been contracted for next year, but had been keen to have his term extended beyond that period.
His sacking continues a period of upheaval at the Saints.
Chief executive Michael Nettlefold recently announced he was quitting, although he will stay until the club can find a replacement.
Summers was recently voted in to replace former president Greg Westaway.
The Saints also lost football manager Greg Hutchison, while it was announced on Thursday that assistant coach Dean Laidley had switched to Carlton.
St Kilda have also traded away their talented young No.1 ruckman Ben McEvoy to Hawthorn to bolster their draft position as they rebuild their playing list.