The real key, is was it salt or fresh water. If salt, it's IMO not worth it the damage can be way to excessive. If fresh, it might just be the $2500.00 cost to repair. More than likely the CCD is fine, it's the controller card.
I know of 2 other IQ backs, that were dropped in fresh water, IQ180 and IQ140, both were on at the time. Both were sent off to Phase and repaired. Cost was in the $2500 to $4000.00 range.
If you do send it off, and Phase fixes it, as already mentioned, it will be fully checked out and re-certified by Phase One as a workable back.
Your bigger issue is the current value of the P45 non + and that you can purchase used P45+ backs for pretty good deals now. And you can pick up a used P45 for even less.
Phase is through with repairs.
Paul C