Part of bone healing after a fracture or joint replacement is that the bone is living tissue which undergoes remodeling according to the stresses placed upon it ; such as gravity. Hence, after some time, the bone, in the absence of complications such as infection, should virtually be as strong as it was pre fracture. In terms of joint replacement, you are correct, the stresses placed upon the joint are quite different, if any, when in the water. In other words, the joint is not "weight bearing" ; a term you no doubt heard form your surgeons during recovery.
As with any trauma, and surgery is trauma, healing can be a protracted event ; especially if there was accompanying muscle deconditioning, or medical issues.
As most of you have already suggested, it will not be the diving that is the main issue but egress and ingress to the dive ; a weight integrated BC may help, for example.
The other issue is, does it constitute an increased risk for DCS? I believe it would as there is an unpredictable amount of scarring in relation to a major joint of the body, associated with healing. How much can't be predicted and therefore would justify curtailing dives as a safety concern.
Overall? I believe diving post joint replacement should be determined on post operative recovery ( how much did range of movement improve, resolution of pain, and return to activities of daily living), overall fitness and any associated medical conditions. And I would suggest being somewhat more conservative, be it incorporating deep stops, shorter dive times etc, into your routine. This is exactly what I did following my own leg fractures.
Safe diving.