"Semper Fi" is pronounced with a long "i", as in "eye" .. . but the entire motto is more properly pronounced with a short "i" as in "fit" or "kit".
Of course then you get into the pronunciation differences between Classical Latin (the "new" pronunciation), the old school Latin pronunciation, legal Latin, and church or Italianate Latin. It's a mess. Just say it however you like, but don't let Ermey hear you . . . . :-D