Locate a suitable long-shaft, waterproof, battery powered motor. I got mine from Canadian Tire. It is shown here:http://www.youtube.com/watch?v=pM7NXDFMhpg
The chair part is useless for most disabled kids as it provides no support, but can be used without the motor so it isn't a total waste. Please note that I do not work for Canadian Tire or Fluid.
I know there are no Canadian Tire's in the U.S., but I would think that many outdoors or department stores would carry the Fluid line of products. That motor is actually used in a few of their products, so should be easy to get.
You will then need to get some pontoons from the old-style water-loungers or something similar, or rig up something yourself. Check garage sales. Make sure whatever you use or make is wide and long enough to ensure that it is stable and will not flip over.
Finally, use the seating system from one of your old wheelchairs. This one is from our daughter's old Kimba stroller. For obvious safety reasons, we removed all of the belts and restraints. By doing this, the seating is still adjustable, and can grow with the child.