The bike is money you can put into the boat and while your knees might turn to custard, the real danger is the loon in the car changing the channel.
If the seat height is set up properly (pain in back of knee - lower the seat, pain in front of knee - raise the seat) and pedaling at a minimum of 80 RPM, you should have no knee problems.
As for loons, they can be found using all forms of transportation. PWCs, powerboaters, clueless sailors, drunk pilots, stoned train engineers.
