My gut reaction was that the horizontal motion (away from narrow inlets) is very small. I tried a calculation. On the US East Coast, St. John's Bay to Cape Cod

My gut reaction was that the horizontal motion (away from narrow inlets) is very small. I tried a calculation. On the US East Coast, St. John's Bay to Cape Cod is about 500km. So, take a 500km long trough that's 1km deep. If the water sloshes so that one end rises by 10m and the other falls by 10m, with a smooth slope between, the midpoint of the body of water would shift by 1.25km, which is more than I expected. But remember that that's over six hours, so, the peak water velocity would be on the order of 2cm/second. It would be pretty hard to capture power from that slow a current.