Well, caisson is from Old French, meaning large box, so I suppose that is why the term came to mean both a structure used underwater and a horse-drawn vehicle used to draw cannon and ammunition. Caisson is also defined as a large box open on one side and the top, designed to fit against the side of a ship and used for underwater repair, as a floating structure used to close off a dock or canal, and as a device used to raise sucken objects. It seems that they are all just big boxes. More on the etymology: the Old French is an alteration of casson, from Italian cassone, augmentative of cassa, box, from Latin capsal.