How far the water actually getsHardly ever the platters, which sit sealed away and read normally afterwards. Most often it's the components outside that seal that get damaged: the contact strip, the connector, the board itself, all of which can be cleaned or simply replaced.
What heat actually damagesMostly the circuit board, and first of all. Platters are capable of withstanding heat that has already melted plastic and damaged solder. Remove the dead board, install a donor, and transfer your own ROM chip from the old board to the new one.
Mains water, dirty water, saltA split mains water pipe is the mild case. Water running across a floor is worse, bringing silt and debris picked up along the way. The worst offender, however, is salt, whose chloride passes straight past the circuit board and into the coating. Whichever grade it was, your drive needs to 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.