The main difference between the two is that cofferdams are temporary where as caissons are built to remain in place as a completed structure. Whatever project is being created will determine which structure is needed. Factors such as ground conditions and the depth of water come into play in deciding the appropriate format. However, both are used when carrying out construction in areas submerged with water.
The goal of a cofferdam is to be as watertight as possible to create a dry environment for construction work to take place. Small amounts of water are allowed to get in as long as it can be pumped out safely throughout the job.
A caisson is a box-like structure made of timber, steel, masonry, and reinforced concrete. The frame is floated to a required location then sunk into place. Remember, this design will remain in the area it is left permanently. After being fastened in place, it is used to access and undertake work.