And just so you don't think I made this up, I did some web research (and everybody knows if it is on the internet...)
I made an initial pontoon from a wooden dowell, "skinned" in Evergreen plastic and filled and sanded to shape. I then made a latex mould and cast some full shape pontoons in Alumilite. Two of the pontoons were used for the wagons and two were embedded into plasticene that was sculpted into a water-wavey surface. Here the single mould, and the original and mould (with plaster back-up--the single pontoon mould was made with heavy string in the brush-on rubber for necessary stiffness) for the dual units:
Here is a sample of the Alumulite dual, floating unit:
The bridge was built in two sections, with a joining piece to link them. I can then use a full crossing or use "two" bridges as an entry was for a game. The full bridge:
For the Union bridge:
Sample boats on display at Tredegar, and I believe near Fredericksburg are standard army green. However, a model at the Chancellorsville visitors' centre shows a boat in white:
I wonder if the model builder here, like me, just decided not to colour the model? In any case I used white boats, and stained the bridge-wood green to simulate a pre-made bridge, assembled by the engineers when required.