I am imagining 3 struts, one each near the sides of the dock with a third off center and close to one of the one of the side struts so you can incorporate your walk way between them. This type construction would only be possible if your shore anchoring points are at or above the high water level. It will probably require struts much longer than 20' so that at low water level the walk way won't be to steep to negotiate.
A possible alternative is to place the shore anchoring points at the mean water level so the struts could be 20' long and having the walkway floating independently of the struts. This way most of the walkway would floating and at the same level with the dock. At low water parts of the dock would rest on the shore.
You could reduce stress on the assembly by arranging flotation blocks in line with the prevailing winds so wave action doesn't have as much surface area to push against.
These are just my ramblings. I am sure more experience dock builders will be along shortly with better info.
