How far the water getsHardly ever the platters: sealed away, they read normally afterwards. Damage lands on the parts outside that seal — board, connector, contact strip — every one of them cleanable or replaceable.
What the heat damagesThe circuit board, first and mainly. Platters come through temperatures that have already dealt with plastics and solder. Off comes the failed board, on goes a donor, and the ROM chip from your own board is transferred to it.
Mains, dirty water, salt waterA split mains pipe is the mild case. Water that has been across a floor is worse: silt, and whatever it picked up on the way. Salt is the hard one, since chloride does not stop at the board — it goes for the coating. Whichever it was, the drive travels damp in a sealed bag.
Soot brings a second roundSmoke does its damage on the way down, settling an acidic, abrasive film into every vent and seam. Turn a drive over in that state and the grit gets dragged across surfaces it should never touch. So: clean first, and keep power a long way back in the queue.