Check out http://www.kayakdock.com/
I have used their products and they are quite stable while providing flexibility for ice and wave survival. I am not familiar with their Tide Manager system and didn't see any info on the website but something would be needed.
In any case I think a floating dock is probably the best solution for big tidal range. I have seen many methods but most revolve around hoops sliding up and down a fixed piling or metal bar . Then the problem becomes getting self and gear down to the floating dock. Ramps tend to take up a lot of room when the tide is high and too steep when it is low but there must be a lot of experience up there with such things.
Our tidal range here on the Chesapeake doesn't exceed three feet so there is much concern here for your problem.