The hard drive itself, is not completly sealed, it has a small breather hole in it. The breather hole has a filter to keep even smaller dust particals out. So althought It looks well sealed up and has seals and is screwed together, it can still be damaged by water getting in the breather hole.
Then apply some other simple sciences , if the drive was heating up, the air inside it would expand some, any mosture in the air inside it would expand even more. If it got "hosed" when it was all hot with cool water, the air would condence , and Tiny bits of water/moisture could have gone into the breather hole.
What to do I dont know? its just that there could be high moisture in the drive now, any Condensation of moisture on the platters themselves could rip the heads right off the armature, on the first spin-up/seek.
I have heard of putting thing in general your trying to dry, into dry rice (weird) or into a sealed bag with Silica packs, those dryers for foods. some way to get things really dry by using stuff that likes to pull moisture away. The silica packs have to be fresh sealed ones or recharged so they do thier job.
The controller board, getting exposed to soot which probably has a high carbon base? , might have to be cleaned completely also, any changed paths could change the resistance and cause the controller to not function correctally.
Magnatism, there are ~3 things that effect magnatism, extreeme heat, extremee shock, and (what was the 3rd) de-polarisation ? From you description , it is likly that the data on the platters themselves still exists.
There are companies that deal with hard drive recovery even for fire, even being experts they would not guarentee sucessfull recovery. I am just trying to think about what could have happend.