The two-out long ball to right field was Posey's first in Hawaii. The No. 5 overall pick in this year's Draft went 2-for-3 with a walk in the game.
Posey hit .385 and hit one home run in seven games for the Giants' Arizona Rookie League affiliate and had three hits in 11 at-bats for the Class A Short-Season Salem-Keizer Volcanoes.
Leadoff hitter James McOwen (Mariners) was 2-for-3 with two runs scored and an RBI and Marquez Smith (Cubs) doubled twice and crossed the plate once for the BeachBoys (20-17). It was the third consecutive year Waikiki reached the championship game.
Southpaw Mitsuo Yoshikawa, who pitched for the Hokkaido Nippon Ham Fighters this season, allowed one run on four hits and two walks, striking out six over five innings to earn the win.
Mariners prospect Steven Richard allowed one hit and struck out two over four scoreless frames for his first Hawaii Winter Baseball save.
Brian Friday (Pittsburgh) singled in Tigers prospect James Skelton in the second for West Oahu's only run. Tyler Henson (Orioles) and Mark Hallberg (Diamondbacks), who both went 2-for-4 were the only other CaneFires (19-15) to get a hit.
Brewers right-hander Cody Scarpetta allowed two runs on four hits and three walks over 2 1/3 innings to get tagged with the loss for West Oahu.
The BeachBoys beat the CaneFires in eight of 12 games this season.
Josh Jackson is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.