A divemaster is no guarantee for personal safety and can give a false sense of security, as we have seen in numerous accident and incident threads recently.
We would consider California diving to be warm water diving. In Ontario, we regularly dive in 38 F water, even all through the summer. There is of course, ice diving as well, so anything above 50 F is really warm. We do not have instructors or DM's in the water or have guided dives for certified divers. For boat dives, the captain will give a dive briefing, reviewing boat safety protocols and the essentials for that dive site. Certified divers need to be able to plan and execute their dives, be self-reliant, be able to navigate effectively and deal with the extra complications associated with extreme cold water diving.
The availability of Dive Masters or Dive Guides are not the answer for most of the accidents we have read about. In this case, the victim resisted the efforts of the DM that was trying to help him, as we've seen before. Training divers effectively and giving useful detailed dive briefings with important information that people need for diving in a particular area would help a lot.