For single-spar designs which are gravitationally stable (C_g below C_b), rather than having the ballast at the bottom of the spar, we could hang it from the spar for additional stability.
You can make more efficient use of your ballast by hanging it lower down and also by using several cables going to a single ballast. If there are several cables, then when the structure tries to tilt the weight of the ballast is shifted onto the side that is trying to lift up. In this case the full ballast weight is trying to pull the high side back down so the structure is level. Here is a video that shows some experiments with hanging ballast to demonstrate the difference between multiple cables to a single ballast and multiple cables to smaller ballasts.
Static stability is when things are not moving, like when we slowly added weights to one side in the previous video. For static stability the length of a single cable does not matter, it is the same as if the weight was at the top of the cable.
Dynamic stability is concerned with stability while moving in waves. The waves can not make the ballast swing back and forth when it is on a very long cable, so it continues to pull almost straight down even when there are waves. For dynamic stability a longer single cable makes a difference as can be seen by comparing the following two videos. First with a very short rope the model swings in the waves:
With a long single line to the ballast the model has very good dynamic stability:
Raising and lowering hanging ballast
Having a long cable to the ballast helps the dynamic stability. However, a long cable makes it hard to get close to land and into harbors. So we might want to be able to winch in the cable when we were coming into a harbor where the waves were small and the water was shallow. Then when we went to deep water again we could lower the ballast for increased stability.
Doing this could add substantially to the cost and complexity of the hanging ballast.
Potential of damage to reefs and undersea cables
A hanging ballast could cause damage to a reef or undersea cable if not used correctly. Boats with anchors can also do such damage and they generally don't. So maybe this will not be a big issue. But we might want something that automatically raised the ballast if the water was getting too shallow, though this has its own risks and problems. If one seastead with a hanging ballast did cause trouble governments might pass laws saying they are not permitted in their territory or maybe in certain areas.
Energy storage in ballast
It would be possible to store energy in the ballast either in batteries or as compressed air. This would let you use the batteries both as energy storage and ballast. A steel ballast could be made like a really strong air tank so that high pressure air could be stored. With these you would need either wires or hoses going up to the living area of the seastead. The difficulty, cost, etc would have to be investigated to see if some sort of energy storage in the ballast was reasonable.
Motors on ballast
Large slow moving propellers could be operated from motors on the ballast. This way they avoid the motion of the waves and would not need to be so strong. Also, if the energy produced from solar panels is stored in the ballast this would be closer to the energy storage location.
So the picture below could be more stable if the cables were longer and they went to a single ballast.