Could you imagine going out for a simple swim, diving down for fun, and stumbling across a car with a rotting corpse in it? There's the reason why I don't like swimming in deep water.
The amoeba is called Naegleria Fowleri and nagelerisis is a terrifying way to die: even with treatment there's a 95% mortality rate.
The amoebas enter the nose when they traverse the olfactory nerve via something called the cribriform plate to enter the brain. Once there, they mistake neurons for bacteria, which they consume.