Joel,
The inside box is the definition for your boat entry slip (to sling lifts I would guess), the foundation for the boathouse structure and the inner wall limit of the rocks between it and the outer wall. The rock mass between the concrete walls is to resist movement induced by anything moving in the river. Some of the very old boathouses in Europe used stone on stone masonry for this two wall system. Going this route calls for individual stones that weigh tons each. The concrete containers is the "modern" approach. Yes I know too that some of the oldest concrete structures have been around for 2000 years (the coliseum in Rome for example).