Building a pontoon boat isn't difficult --- if you pre-plan the project properly --- and the result will provide many hours of recreation for a family. Boating also helps build teamwork, teaches "real-world" skills, and means quality time for family members. Issues like flotation, maximum number of passengers, insurance and legal compliance mean this is a major project; even so, it can be done in the five steps that follow. Building a 20-foot pontoon boat requires the same skills as building any other boat --- or a house or garage. Putting these skills to work for a recreational result can be as rewarding as the boating itself.
How to Build a 20' Pontoon Boat
Develop a design. This seems obvious --- pontoons, a deck, motor and mounts, fuel system, water system, steering system, handrails, lifejackets and lifejacket storage, deck gear (cleats or bits to tie the boat to the dock, an anchor and anchor rode, anchor storage), a bathroom and storage for miscellaneous stuff like fishing gear. Your only limit in the design is your imagination, as long as you comply with Coast Guard safety requirements. The biggest issues you'll face are safety and flotation.
Determine the weight of the materials you will use to construct the boat, the weight of the passengers and the weight of systems such as the motor, power source, steering gear, the head and so forth. Everything that's included in the design, plus passengers and the stuff they bring with them, has to be included to determine how much flotation material is required. Weights of materials are available from the the suppliers where you purchase the materials and equipment. Allow an average of 225 pounds per person and gear for each potential passenger. The pontoons of a pontoon boat are not completely empty. If they were empty, and the boat were to hit an underwater obstruction that poked a hole in a pontoon, the pontoon would quickly fill with water and the boat would sink. The best foam for use in a boat will be resistant to water and solvents like gasoline. Styrene acrylonitrile, an improved foam with good floatation characteristics, is probably the best for this application. It weighs 4.0 pounds per cubic foot, meaning that, in fresh water, where the water weighs about 62.5 pounds per cubic foot, it will provide 58.5 pounds of flotation for every cubic foot of foam used. Because you know the weight of the boat, its occupants, and its equipment, it will be easy to calculate the number of cubic feet of foam that will be required inside the pontoons for effective flotation. After you have determined the quantity of foam required, you can place it between the inside and outside skin of the pontoon: a full double hull is not needed; just a bit of sheet metal work to enclose the foam. The air in the rest of the pontoon will contribute to the flotation provided by the foam and provide a margin of safety.
Plan the construction of the boat carefully. Nearly everyone has heard the story about the fellow who built a boat in his basement, then had to tear down a wall to get the boat out. Don't forget to plan on a trailer as part of the design and construction effort. The pontoons are, arguably, the most important of the large systems in the boat and you'll probably spend more time on them than on any other system. They're also the only part of the boat that you will need to test for leaks before construction is completed --- you can do this at a local lake.
Assemble the sub-systems for the boat: the pontoons, the framing to support the deck, the deck and associated equipment, including the electrical and mechanical components, like the wiring, the steering gear, the fuel system, the handrails and the motor mounts.
Assemble the boat. Most people don't have ready access to a crane, so the order of assembly should probably follow this pattern: with the pontoons positioned on jackstands, bolt the frame to the pontoons. Then put the pontoon assembly onto the trailer, which you will use to carry the vessel to water. From that point, you can add other sub-systems (deck, steering, motor, etc.) as time, energy and money permit. Don't forget to take along some rope for mooring and docking, and remember that all states, as well as the U. S. Coast Guard, require one lifejacket per person on board.