It all depends on how much water you want to boil and why. I see a few solutions, but to boil any real amount of water (say over the tablespoon amounts) you really need another source of low grade heat, like a loop of copper pipe attached to a solar hot water tank. Take the outputs from the generator and wire them to thermoelectric elements (like those giant 350watters seen all over ebay), sandwiched between the warm water loop from the tank and your small tank of water to be boiled. You might need some control electronics in between, I've read that spikes and dips are bad for the peltiers and shorten the lifespan.
The second option I see would a stirling engine used in reverse as a water heater, physically attached to the crank of the bike (in place of the generator), you would also need a source of low grade heat, like a solar warm water tank.
If you're looking into distilling water, then something like a reverse osmosis filter might work better for you, you might be able to use a water pump to push the water through instead of using the energy to heat the water.
What are you doing with the water?
Thanks, Dave