For buoyancy, I decided to use a lot 1 gallon milk jugs to keep the cost down. According to Archimedes' Principle, the weight of the amount of liquid an object displaces is equal to its buoyancy. Because I weigh about 175 lbs and I estimated the bike to be between 35 and 45 lbs, I will need a total of 250 lbs of buoyancy. This is 32 gallon jigs.
In order to get all of the materials I needed, I kept it cheap. The bike I got from a friend, but there are always cheap bikes available at garage sales. I used old tent poles as support. Duct tape and twine are relatively inexpensive. My biggest purchase was the wood that I needed, which came out to only $20. The largest purchase I made (well it was free so it wasn't a purchase) was getting all of the bottles from the recycling center. Get ready to go dumpster diving!
Riding the bike on land is no problem if you put the floaters far enough in front and behind the pedals. Then, it is just a matter of getting used to the additional structures on the bike. Mine, for example, is not able to turn and pedal at the same time because of the floaters. Using twine or something simple will be enough to keep the floaters from hitting the ground.
One of the most complicated parts of the build was keeping the floaters down while in the water. Because about 250 lbs of force will be pushing up I needed very strong supports. Unfortunately I only had tent poles, which were strong enough to keep only the bike buoyant. If I reinforced these with additional poles, the bike would be able to support myself also.
The directional system in water is just the same as it is on land: turning the handlebars. Once moving by pedaling, that will allow the bike to turn while in the water. Because of the enormous amount of upward force present in the water, I was not able to test the bike with myself on it. However, if you wanted to adding a rudder wouldn't be a big problem.
The propulsion system is lacking, but while pedaling the rear wheel will allow for some forward momentum. I had an idea of attaching a wheel to the rear wheel and water wheels to that, so that when you pedal the water wheels will propel you forward, but only in the water. However, you would need two wheels attached the the rear one if you want to go forward when you pedal forward!
As a prototype, this design worked very well. With only a few minor adjustments, this will be able to support a person on the water and be able to go forward, however slowly. Feel free to duplicate this but be sure to let me know and post pictures so that everyone can see!