Is the kill switch that has been mentioned a tether like on a SeaDoo that kills the motor if you're ejected, or something else?
I still like the hand signal or flag start given by a third party for two reasons:
With two evenly matched boats, the driver giving the nod will always have a slight advantage, and;
Instead of looking at each other, the drivers will be looking forward at the river in front of them.
A hand/flag start would require more people to stage a race though.