In reply to FesteringSore:
The RSPCA are one of several organisations who have water-rescue capability (SRT training, the same kit as Fire/MRT/RNLI/AA/Others) and work to the same set of guidelines and protocol as the other rescue agencies. As some have pointed out, the main reason for this is that the RSPCA have more experience dealing with animals.
All of the above may register as a DEFRA asset. You get a certain amount towards training costs and in return you may be asked to assist with major incidents. As everybody should have similar kit and training standards, you can mix and match personnel from different agencies to make up a team/boat crew. It can be quite useful if you need to deploy specialists to different locations...
The downside is that you have to wear the full kit even if you are stood in 2 inches of water, so it looks a bit like overkill...