I saw it as a typical bubblegum, Saturday matinee movie, but with a better than typical cast, including the recently late Maximilian Schell (d. 1 Feb, 2014).
I think its biggest problem was the underlying mood of the situation, which they approached as a typical 1960s Hammer studio science fiction, instead of the much broader look, like in Crack in the World (1965). Because Krakatoa was a planetary event, it didn’t really lend itself to small studio work.
Read Simon Winchester's book about Krakatoa's 1883 eruption, a few years ago and really enjoyed it. Other than the incredible incidental stories, boats ending up miles up stream, a half mile above sea level, etc., his thesis held that the eruption (August 27, 1883, BTW), was the first international disaster to become instant news throughout the world, following the laying of telegraphic cable worldwide.