Alonso, who had six hits in 19 at-bats for the Florida State League's Sarasota Reds in 2008, boosted his Hawaii Winter Baseball batting average to .312 with his 3-for-5 day at the plate.
The BeachBoys (15-12) were down by a run going into the ninth inning, but Marquez Smith (Cubs) cranked a solo homer to tie the game. Giants first-round pick Buster Posey singled and advanced to second on a wild pitch before Alonso plated him with a line drive to center field.
Posey, who was 0-for-4 coming into the inning, extended his hitting streak to 11 games.
Kazunori Yamamoto (3-0) of Japan's Hokkaido Nippon Ham Fighters allowed two runs -- one earned -- on two hits and two walks over two innings for the win. David Pfeiffer (Dodgers) tossed a perfect ninth for his third save of the season.
BeachBoys starting pitcher Waldis Joaquin (Giants) gave up three runs -- two earned -- on four hits and four walks over six innings. He struck out two.
West Oahu's Caleb Gindl (Brewers) amassed his first multiple hit game since Oct. 10. He doubled, singled and scored twice.
Gregory Burns (Marlins) stole his league-leading 11th base for the CaneFires (13-14). Burns also walked twice and scored a run.
Orioles prospect Clifford Flagello (1-2) was tagged for the loss and his first blown save in Hawaii. Starter Chris Cody (Brewers) lasted five innings for West Oahu. He gave up four runs on six hits while striking out six and walking two.
Josh Jackson is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.