This may be because none of them are active. Use Ctrl+RMB without pointing to a layer to make all layers visible. If you still do not see anything in the 3D view, try the FrameModel button in the graphics toolbar .
The Frame model button.
Another reason could be that you have a model with a completely flat seabed surface, and you are viewing it from above. This problem typically occurs with single-layer drag&drop models. Resolution: tilt the model slightly and the layers should become visible.