The Giants media guide lists every Splash Hit in the two-decade history of Oracle Park, and it's very specific about what the rules are. Above the list of 130 home runs the organization has counted as Splash Hits is a sentence that makes it clear they "have been hit over the right field wall and landed in McCovey Cove on the fly."
Thus, Mike Yastrzemski lost his first career Splash Hit on a technicality. Yastrzemski blasted a Chris Paddock changeup 379 feet in the third inning, but before it hit water, it glanced off a pillar designed to shoot water into the air after home runs.