Brian,
Interesting challenge!
I recommend from experience, you use 4 off 2 way pumps, each connected to an independent chamber.
4 chambers, in the bottom of the dock, subdivided f/a and p/s each with baffles to reduce the free surface effect, this will allow you to trim the dock as it sinks/rises.
Keep ALL the pumped ballast in the bottom of the dock, use the sides as buoyancy - you WILL need them otherwise the whole thing will become really unstable and liable to capsize without even thinking about it - read my Blue Marlin build for my experience of this.
Fill the buoyancy chambers with polystyrene so that if they spring a leak you preserve buoyancy.
Dont make the mistake I did by using free flood ballast, it drastically reduces the available lift of the structure - make it all pumped ballast.
6mm ply will be plenty man enough for the job, just ensure everything is well sealed against the water.
If you lift the pumps above the waterline, the pumping rate will be reduced, if you can do it, keep them as low as possible, best would be in an airtight tank in the bottom of the structure - again, look at the Marlin build to see how I did it.
Ian