The pre-requisite to start the Divemaster course in only 20 dives and Rescue Diver. So, don't overestimate the general level of ability of DM Trainees.
However, I think some sort of general diving ability should also be present....an assessed 'check-out' dive with the instructor concerned. Even then - a good instructor can work wonders with a DM Trainee over a period of months. What counts is the final product.
I have a lot of issues with DM training - as I feel the standard varies too greatly....
1. 20 Dive minimum to start. Too low. By the time OW (4 dives), AOW (5 dives) and Rescue (4 dives) are completed, it only leaves the diver with a requirement to complete 7 fun dives. I think any diver should need to have more time in the water to develop their own interests and love for the sea, before engaging on a dive leadership program.
2. Taught by Open Water Instructors. Whilst there are some hugely experienced Open Water Scuba Instructors working in the industry, the current system allows people fresh off an IDC to conduct DM programs. I feel that professional level programs should only be taught by more experienced instructors. If PADI raised the bar on this, it would add more credibility to the MSDT rating. Or even shift up to IDC Staff level. This would be simular to SSI, who have the Dive Control Specialist Instructor rating.
3. Too little emphasis on general dive ability. Whilst the DM program does have a system of assessments, there is no specific assessment for the candidate's overall ability and aptitude in the water. Bouyancy, Trim, Risk Assessment, Dive Planning....are not covered adequately.
I wrote a distinctive speciality for an Advanced Diver, but I never got around to submitting it to PADI. It basically covered simular ground to the Fundies course, with additional skills such as DSMB deployment, Basic Line Laying, More Complete Deco Theory, More complete First Aid (especially barotrauma, DCI and marine life injuries), Strong Current Diving, Low Viz Diving, Gas Redundancy, Gas Planning, Intro to Tech, Intro to CCR.
It was designed as a stepping stone towards Tech Training and/or to develop truly capable Divemasters.
In the end, I just chose to include these elements within the normal training system - adding them as extras in existing courses and/or the DM program.