Harold Holt was Prime Minister of Australia which, while making him more interesting than most people, doesn't make him all that special. What really makes him unusual is that in 1967, whilst he was still Prime Minister, he went swimming and did not return. His body was never found and he was assumed to have drowned.
What was more interesting was that anybody cared. After all, we know that if you're the President of the USA you can blow your nose and you'll get publicity, but Australia's situation on the world political scene is not exactly what you'd call superpower-style. People just don't care. Just look at Bob Hawke's1 wife. She's selling painkillers on television: apparently people are supposed to trust her, even though few trusted her husband during his time as PM. So, for anyone to care that Holt died, even when his body was missing, was just not normal behaviour for the Australian populace.
Another striking feature was the volume of conspiracy theories that arose from Holt's disappearance. Some people said that he was troubled by his political problems, rumours about the marriages of his three sons and the death of his brother, Cliff. Other rumours say that he wished to uphold his premonition of not living beyond 60. However, many of the theories revolve around an abduction by Russian - or sometimes Chinese - agents in a submarine, or indeed that he was a Communist spy and went willingly.
The Soviet spy theory has some merit, as his disapperance occurred during the Vietnam War and being the Australian Prime Minister he may have been in a position to find out about US military plans. Of course, since the US probably didn't really care about Australia's meagre involvement, he might have had to read about them in the papers just like everyone else, so on reflection this theory has no merit.
There could well be a simple explanation for his disappearance. The sea next to the beach where Holt was swimming is shark infested. It's bloody dangerous. What's more, looking at photos of the man, you'll see that he wasn't fit enough to clear the waves' break. It's just not viable. And the lifeguard certainly wouldn't have made it through to pick him up. So, as the official inquiry concluded, he probably just drowned and became a statistic in the Surf Lifesaving Annual.