The pontoons are big, flat metal boxes with cables to keep them in place. The superstructure is attached to the pontoons with tubular steel members arranged in a 'W' shape. The superstructure has a deeper section at midspan supported by little cable-stayed towers over the pontoons. The approach spans are supported by simple through trusses. My son Aaron can be seen in this photo standing on the deck.
Pontoon structures are an interesting class of bridges. They are a good idea in theory, but they sometimes have problems in practice. The first recorded use of a pontoon bridge was in China in the 11th century B.C. They are often used by the military as a quick way to get across a river. The Persians apparently liked them and built famous pontoon bridges to pursue the Greeks across the Bosporus and across the Hellespont. Usually they are carried by an army and removed after they have crossed (to make it harder for the enemy to follow). My experience with them was occasionally driving on the Lake Washington Floating Bridge in Seattle before it sank during a storm in 1990.
Great Britain: Docklands Pontoon Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.