Argument: Tidal power has a low capacity factor (poor return-on-investment)
"PROS AND CONS OF TIDAL ENERGY USE". HYDRO ENERGY FROM THE MOON". Energy Consumers Edge - Tidal energy projects involving tidal dams are more expensive per KW of installed power than similar size systems that use river dams. Tidal flow is intermittent. Twice a day tidal flows go through a flood stage, slow down, stop, reverse into an ebb tide, slow down, stop, and repeat the cycle.
This constant start and stop cycle creates intermittency problems similar to wind turbines and wave generators. Though a tidal dam might be identical to a river dam in every way including cost; the tidal dam will produce less than half the amount of electricity.
A typical average plant load factor for tidal energy generators is about 27%. Load factor defines the amount of actual power output expected from a given capacity. Installed generating capacity of 100 MW with a load factor of 27% would produce only 27 MW per hour when averaged over a given time, usually a year.
That makes tidal energy expensive.
This U.S. Department of Energy Tidal Energy Report concludes that tidal power costs are not competitive with fossil fuel plants.