Hi,
Norm put a lot of thought and work into his scheme for supporting and separating the copper hx coils in his tank:
Solar Hot Water and Space Heating System With Integrated Boiler
I've used plain old polypropylene rope, but in my case the hx is pex and its pretty close to neutral buoyancy. But, the rope was still in good shape last time I opened the tank -- probably after about 4 years.
Gary