Very important to have a dry field. Doc should have hemostatic agents for this type of situation. Somewhere I have x rays of two different patients
who had deep decay nearly to the crest of the bone, perfect margins. They didn't want to extract and they didn't want an implant. They were prepped to the bone level
at least 6mm sub gingival. Tissue was retracted with a few cords and an IOS was taken. If you can see the margins with your eyes then the IOS can capture the data.
If not, the advantage would be squirting impression material down there.