The greatest concern for any type of exercise after hip or knee replacement surgery is for the surgical site and bone to be completely healed. When your doctor releases you for full activity, and you are able to perform your exercise and daily living activities with full weight bearing and no difficulties, such as pain, swelling or stiffness, diving would probably be allowed.
There isn't much difference between the physical demand placed on your hip or knee during a workout in the gym and during scuba diving. As long as the area has an adequate blood flow to help with the off-gassing of nitrogen, a problem is unlikely.
As with any type of injury and returning to diving, there is no guarantee that you will not have an injury to your hip or knee. However, the greatest risk divers run is the amount of exposure (depth and time) they have to nitrogen. It might be wise to dive conservatively by not diving as deep, as long or as often when you return to diving.