Bathymetric mapping techniques using either sonar or the interesting "rope with a weight on the end" method you mentioned do not capture overhanging features well. You would need to collect 3-D data like with an AUV or ROV to produce a model, and a standard bathymetric projection map would not be able to display it in any case.

I have been in a submersible and encountered large overhangs not shown on maps even in areas that have extensive data. It's unnerving.