Might not be such a good idea?
On the face of it then it sounds like maybe it could be a good idea. The only other issue that perhaps crops up is that vibrations from hard drives are going to efficiently transmit via liquid to other hard drives as shock waves as well as to the rest of the computer hardware also in the liquid, it wouldn't look too good if say for example after a year or two you suddenly got metal fatigue in the wiring on the motherboard leading to micro-fractures, cracks or dry joints immediately filling with the non-conducting liquid causing electrical problems.
This is a issue that hasn't existed before because everything else previously immersed never had any moving parts. You would have to research and test this aspect thoroughly before you give immersed hard drives the green light but I suspect what will happen is that someone will blindly go gangbusters with this idea until major failures occur a few years later and then everyone will be none the wiser as to what caused the problem.
The possibility also exists that this may totally be a non-issue but given that the stuff that's immersed is usually very expensive I wouldn't want to blindly bet on it being the case.