. . . by ignorance. You know more about dry-ice (solid carbon dioxide) than you think you do.
1) it is cold - you have to get CO2 down to -78°C before it freezes. It is therefore useful for shipping stuff cold - biological samples or high-end meat. Regular ice leaves puddles behind.
2) it sublimates - avoiding the liquid transition between solid to gas that most other chemical compounds experience. When sublimation happens a white cloud fills the air and wafts across the floor - this makes good theatre in talent shows or for Macbeth's witches.
If you have read The Blob from the beginning and can remember it all a) you're doing better than me b) you'll know that CO2 can be deadly: it killed 1700 people and countless animals when Lake Nyos erupted at midnight in 1986; it can also terminate mice inhumanely; one of my colleagues wasn't killed by the stuff - but it was NDE close.
Some clever clogs in Russia obtained a 25kg boxful of dry ice and threw it in a swimming pool for a birthday jape. What could possibly go wrong? It should be okay because the swimming pool is outside and CO2 is a) gas b) heavier than air. If everyone stays standing and there is the slightest breeze and/or the ground slopes away in any direction then there will be calm after the foggy sublimating excitement. This argument is similar to that where short people [aka children] were disproportionately killed when the Syrian government lobbed the nerve-gas Sarin into the rebel-held suburb of Ghouta in August 2013.
It would be terminally foolish to dive into a swimming pool buried under a fogging blanket of carbon dioxide; but 3 people at the birthday party elected to do that and had no more birthdays. Sad really; really sad, so young and full of promise. It is possible that alcohol was involved because a) it was an adult birthday party b) that has a huge capacity for making people do things that seemed a good idea at the time.