Stephen Ruiz fired Tower ahead early on after Grouville goalkeeper Casey Hickling had made a good block following a free kick.
Influential midfielder Liam Walsh bagged his 11th goal of the campaign to draw the scores level, before Chris Carter fired Grouville 2-1 ahead after scoring direct from a corner kick.
There was little between the teams in the second half as the heavy ground made good football difficult to produce.
Grouville’s victory was the seventh on the trot and the drainage improvement carried out last summer by president George Noel and helpers was a big factor in enabliing the pitch to be fit for play.
St Paul’s paid a price for hitting the woodwork twice in the second half, through Craig Russell and Craig Leitch, as they draw 2-2 at St Peter.
Last year’s champions scored equalised with the last kick of the game deep into time added on for stoppages.
St Paul’s frustrations led to verbal confrontations with referee Mark Le Cornu, who confirmed that he would be reporting manager Craig Culkin and defender Shaun Irving for comments made after the final whistle.
Junior Jason De Andrade, who again showed up well on the left, fired St Paul’s ahead with a crisp shot that St Peter goalkeeper Gary Lloyd got a hand to but could not prevent entering the goal.
Jack Cannon equalised with a fine low strike after an excellent cross from St Peter’s right by Ryan Durand.
Lloyd made a good block to deny Jordan Docherty, but he could do nothing with the rebound as Russell swept the ball in for his tenth goal of the season.
St Peter, who were second-best for long periods of the second half, pressed forward in the closing stages and although Andrew Bird blocked substitute Aaron Parkinson’s shot, he could do nothing about the youngster’s follow-up attempt.
Jon Trigg and Martin King’s St Peter are now locked in a relegation battle, having scored just five goals in six games and collecting only four points.