ACA Safety & Rescue instructors have a unique challenge, they need knowledge of safety aspects for all types of craft like kayaks, canoes, raft, and SUP. Each of these craft have unique safety implications. If you haven't done so, try paddling all of these craft to gain some familiarity - by the way, SUP is a great deal of fun.
The major difference between the class levels is duration. I feel it takes a full two days (Level 4) to cover this subject adequately with lots of hands-on work. The lower level classes concentrate on rescue basics such as throw rope usage, prevention, swimming, wading, etc. Levels 4 and above get more heavily into gear, especially the rescue vest, knots, and mechanical advantage systems.
Safety & Rescue instructors also need to know basic self-rescue skills like the roll, swimming with and without gear, etc. All need to be masters in throw rope usage. We also expect extensive knowledge of river hazards and how to mitigate.