The problem is the weight of the cable. You can reduce the effective weight of the cable in water by, for example, combining polypropylene and steel strands (t

The problem is the weight of the cable. You can reduce the effective weight of the cable in water by, for example, combining polypropylene and steel strands (they experimented with this for some anchor cables in the late 70s early 80s) but the cable has larger diameter with greater drag on the moving robot. Maybe even enough drag to snap the cable anyway. Also, I am not sure how well the experiments turned out, I can see several other possible problems with the combined cable.