Re-read mizu no sho - the book of water
I think what Musashi was trying to explain that one needs to be calm (living calmness that is) and not to take any particular stance.
Musashi believed that we should always be calm in any way. Wether we are in a fight, walking, eating, sitting, and so on, our state of mind should always be calm but ready.
Well, what Musashi explained actually corellates to Ki. But, he did say that he didn't want to use anything from an esoteric Buddhist scripture or anything too spiritual. That's why he uses examples, like those stances. What those stances are, does not actually matter in getting to his point.
In Aikido, there are no actual fixed stances, one only needs to be balanced, centered (one point), and unified (mind and body). Understand this, and the stance that one needs will be revealed.
But if you must know what those stances are, this is what I think (I'll use the terms that Ochiai Hidi sensei uses):
Jumping Footwork - If you ever seen a Tae Kwon Do competition (since I was in TKD myself back in the past, ITF and WTF), you will see that the person does not stand still, but bounces in position. The reason for this is that it is for easier reaction. In TKD, it is believed that if you stand still, it will be slower to react. This is true at some point, since TKD uses a lot of kicks.
Floating Footwork - When you see a Kendo competition, you will see that they look as if they glide on the floor towards their opponent. I believe sliding (ashi sabaki) is what was meant by the floating foot.
Static Foot Position - This sounds like Sumo to me. One stands their ground, becoming a mountain. A mountain does not move. This position is opposed to the other two.
I hope, by this, I had any meaning in what I've just wrote.