You would be surprised at how much changes in the vacuum environment in an orbit around a star. Suddenly the whole tether needs to be insulated, lest whatever working fluid you carry freezes in the shadow or boils in the sunlight. The insulation needs to survive flexing in those temperature extremes. Earthbound liquid ocean environment is quite thermally benign - it is all within the confines of liquid salty water. Almost none of the non-metallic materials used in this 100+ year old pre-SCUBA technology would withstand the environment of space for any usable length of time.