The victory pulled Honolulu (17-17) within a half-game of first-place Waikiki (18-17) in the East Division heading into Saturday's regular-season finale. The winner will face West Oahu in Sunday's Championship Game.
Nagai (4-2) made just one mistake, serving up a solo homer to Giants prospect Roger Kieschnick in the second. He issued a walk to Reds first-round pick Yonder Alonso in the seventh but was otherwise perfect as he tied West Oahu's Chris Salberg for the league lead in wins.
The 24-year-old right-hander, who pitches for Japan's Tohoku Rakuten Golden Eagles, also leads Hawaii Winter Baseball with 48 strikeouts.
Mets left-hander Roy Merritt followed with two perfect innings, fanning two, to earn his second save.
Braves prospect Kala Ka'aihue homered in the eighth to give Nagai and the Sharks some insurance. Domenic Brown (Phillies) was 3-for-4 with a pair of doubles and a run scored for Honolulu, while Lars Davis (Rockies) contributed two hits.
Kieschnick's homer was his sixth of the fall, tying him for the league lead with North Shore's Kyle Martin.
Yankees prospect Jeremy Bleich (3-2) allowed a pair of unearned runs on five hits and three walks with two strikeouts over five innings for the BeachBoys.
Josh Jackson is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.