Hi,
Interesting idea.
I tried to duplicate your calculation of the energy potential for recovering the tidal rise energy in a 25X40 ft patch of water using a different approach, and got a different answer.
If you think in terms of capturing the rising tide in a 25 ft by 40 ft pond (instead of using the 25X40 float), then, when the tide is out, you run the contents of the pond through a 100% efficient turbine to generate power. This makes use of 100% of the tidal rise energy and (I think) should be equivalent to any other scheme that makes use of all the tidal energy (like a float driving a geatrain driving a generator).
If the total tidal rise is (say) 6 ft, then the potential energy you capture in the pond would be:
PE of water in pond = (25ft*40ft*6ft)(62 lb/ft^3)(3 ft) = 1,116,000 ft-lb
Where the 3 ft is the average height of the water in the 25X40 pond.
In Kilowatt-Hours (KWH) this would be (1116000 ft-lb)(1 KWH/2655000 ft-lb) = 0.42 KWH
So, it seems to me, that for your scenario of capturing the energy in a 25X40 ft patch of tidal rise, you would get more like 0.4 KWH per tidal change -- assuming a 6 ft tidal rise, and 100% efficient conversion of the tidal energy to electric energy??
Gary