Since the sport of scuba is self regulating and non policing I doubt you’ll get any traction on a minimal fitness beyond that which we already have.
For those that have forgotten we do have some fitness standards. We test divers during their OW certification. Float/treadwater, Swim/Snorkel and Tired diver tow. We also have similar standards to join the professional ranks but we make it a snorkel and a swim, increase the difficulty and time the results. Since all certifications are lifetime certs oce meeting these standards it is up t the individual to maintain condition.
The required skills an instructor must impart to a student for that student to meet requirements for certification does not require an instructor to be in the shape of a well trained athlete. The instructors responsibility is to pick conditions that will allow them to safely train the student. If this part of the instructors job is done well the condition of the instructor will have little to do with the ability to keep the student safe. For advanced students that have the basics mastered the intructor’s physical condition should be less significant (as long as the dive profile is within the instructors ability) as each diver (both student and instructor) are responsible for their own safety and the instructor is there as more of a demonstrator for the student and judge of the students skills and finally his buddy.
As a dive professional I can assure you that I certainly will not be taking a diver on any dive before they have shown me they are ready for the conditions that can be expected. I certainly will not be taking divers on any dive that I have questions about my abilities to complete sucessfully. I would hope that nearly all dive professionals would approach the safety of students and their fitness the same way.