This song is about Satan, and his descent to Hell (his "dive"). The lyrics deal specifically with Revelations 12:9 where Satan was cast to the earth. The tiger is symbolic of the wild beast that comes out of the sea, as mentioned in Revelations 13:1. (thanks, Light - lanburry, England)
Dio is Ronnie James Dio, the replacement vocalist for Ozzy Osbourne in Black Sabbath and the original lead singer in the band Rainbow. His solo career was far less successful than Ozzy's, and there was some animosity between the two, but when Dio died of cancer on May 16, 2010, Ozzy released a statement saying: "I was very saddened to hear of the passing of Ronnie James Dio. Metal has truly lost one of its greatest voices."
In June, 2009, Dio told the NME: "'Holy Diver' is the metal tune that will not die. For some reason, that album and the song really hit a chord with people. Killswitch Engage did a version of it and I saw them do it at a festival recently where the reaction to their version was wonderful. But it's not something that gets resurrected every once in a while, it's just always there. I guess it's because good songs always work. What usually happens when people in bands meet me for the first time and they're fans is that they'll sing [adopts a self-mocking high pitched voice] "Hoooly Diiiiiver" at me! It happens all the time and it‚s very touching.
This song was featured in an episode of South Park where Dio (in crude cartoon form) appears performing at an elementary school dance with a monkey on drums. In the same NME interview, Dio said: "When I heard South Park wanted to use 'Holy Diver‚' I was like No way! You're gonna completely destroy me, I'll be so embarrassed to see it. But they said they wouldn't because they were big fans and they didn't. They were very kind to me - not so much the drummer - but very kind to me! Plus, if you want to be an icon of some kind, you have to be in an episode of South Park." (thanks, DeeTheWriter - Saint Petersburg, Russia Federation, for above 2)
The album cover depicts Satan whipping a priest with a chain, with the priest shackled and drowning in a turbulent sea. Predictably, this caused some controversy.
This was covered, ironically, by the gospel singer Pat Boone on his In a Metal Mood album. Acknowledging the subject matter, the song ends with Pat reciting Psalm 23:4. Dio sang the backup vocals - he thought it was hilarious. (thanks, Brett - Edmonton, Canada)
In 1989, Japanese video game company Irem made a game based on this song, featuring a protagonist who resembles Ronnie James Dio. The game was never released outside of Japan. (thanks, Douglas - Waterloo, England)