How far the water actually getsHardly ever the platters, which sit sealed away and read normally afterwards. It is the parts outside that seal which take it: the contact strip, the connector, the board — cleanable or replaceable, every one.
What heat actually damagesMostly the circuit board, and first of all. Platters survive temperatures that have already finished off plastics and solder. Off comes the dead board, on goes a donor, and across to it moves your own ROM chip.
Mains water, dirty water, saltThe mild case is a split mains pipe. Water that has run across a floor is worse, bringing silt and whatever else it gathered en route. Hardest of all is salt, whose chloride goes straight past the board and into the coating. Whichever grade it was, the drive should travel damp inside a sealed bag.
Soot causes a second problemSoot damages on the way down, leaving an abrasive, acidic film in every seam and vent. Turn a drive over in that condition and the grit is dragged across surfaces that should never be touched. Clean first, then, and let power wait its turn a long way back.