The Mariners were in control against Gosnells but unusually, after the re-scheduling of four Round 1 matches, all eight Round 3 washouts will not be replayed.
After two losses and the draw, this leaves the Mariners like a shag on a rock and desperately needing to bank some wins, starting with Bayswater-Morley this weekend.
Jayden Derosa held things together early at Sutherlands Park, with the opener making his second first grade half-|century.
Diamanti, the former New Zealand international, took over and made an even 100 off 122 balls with seven fours and five towering sixes.
He was able to launch late because Elliot Lyons (19) became a willing ally in a seventh wicket stand of 78 that pushed Rockingham-Mandurah to 7-217.
Gosnells faced three overs but Diamanti’s first century would amount to naught.
Rebecca Edwards bludgeoned Subiaco-Marist’s bowlers to guide Rockingham-Mandurah to a stirring win in female B grade T20 action.
Edwards (38 off 32) proved the difference with four boundaries and a long bomb to get her team home with an over to spare.
Earlier, Mel Saunders (3-7) and Jennifer Brown (0-7) bowled four-over spells that kept Subiaco-Marist to 3-87.
The third and fourth grades were in the battle, if not slightly ahead of the game, before the Saturday rain persisted.
Thirds had Gosnells 2-12 after posting 9-150 from 45 overs, while teenager Lachlan Pratt produced 51no in the fourth grade’s score of 6-160.
Jamie Ryan took three catches and claimed 2-33 from 10 overs to be the shining light for second grade, who were saved from attempting to overhaul Gosnells’ large total of 5-268.
On Saturday, the firsts and thirds will play at Lark Hill as the seconds and fourths travel to Hillcrest Oval for the last of the four one-dayers that have started the 2013-14 WACA season.