How far water getsRarely as far as the platters. Sealed in, they come out of a dunking still readable. The damage lands on everything exposed — board, connector, contact strip — and each of those can be cleaned up or replaced.
Where fire landsThe board, mainly. Platters ride out temperatures that have already destroyed the plastics and melted the solder. Off comes the burnt board, on goes a donor, and your own ROM chip moves across with it.
Mains, flood and saltClean mains water from a split pipe is the kindest. Floodwater is a step down, carrying silt and whatever the drains were holding. Salt is the worst of the three, because chloride does not stop at the board — it works on the coating. All three go in the bag wet.
Soot is the second hitThe second round of damage happens as things cool: smoke settles out into an acidic, gritty film that finds every seam and vent. Spin a drive up in that state and it drags the grit over its own surfaces. Cleaning comes first, power much later.