Your formula is : E = 0.5 * A * p * h^2 *g. All the numbers you have plugged in appear to be OK except one. A, the size of tidal barrage is 56.5Km^2 is way too big. It could be 5.65 Km^2 =5.65e6. So that you total will be:1,817,989,200,000J. Now you have 2 high tides per day.So, 1,817,989,200,000J x 2 =
3,635,978,400,000J. Therefore, the mean power generation potential = Energy generation potential / time in 1 day =3,635,978,400,000J /86,400s =42,083,083 / 10^6 =42 MW - per day.
Sorry, that is the best I can do. The only Math error in your calculations is 56.5Km^2 =5.65e+7 NOT 8.