Talking about the rapids, Aren't they just the red boats with a yellow cover put over them with the seats removed? Thats what I always thought as there is always some red still visible.
The rapids boats are in two parts - the plastic shell and the rubber float. The rubber floats are original, the shells were the bit which was replaced. I think there's some red on there because at some point (when they still had the comfy red shells) they were resprayed with the float attached.
They did take a while to introduce - I seem to remember in 2001 there was a trial with a 9-seater boat, then one appeared in 2003 and there were still red ones about when the Flume got its retheme. At the time it was massively unpopular - aswell as being bloody uncomfy, to start with they didn't even have a grab ring! Many feared that it meant the Rapids would loose their edge (as Thorpe's Thunder River had undergone a similar transformation coined the Drybena Rapids).