Here is my experience with medical forms.
Based on what I read in this thhread, my friend and I got knew doctor signatures on a PADI form. We first did a day of diving with Poseiden out of Port Douglas. We had to fill out a form that included about 5 questions related to medical issues. We produced our forms, and the guy taking our info, apparently new, was flustered. He had to ask a more senior crew member what that was all about.
The next day we got on our liveaboard, where all we had to do was sign a single statement declaring we were fit to dive. Having brought those signed doctor statements, we produced them anyway. Again, they were surprised to see them. We clearly didn't need them.