Hi Jack. That's a fair point. All turbine-generated power—wind, tidal, air pressure, etc.—remove momentum from a mass in order to turn it into electricity. Motion energy is converted into electrical energy and therefore the energy is removed from the ocean. And, no conversion of energy is perfect, so some energy is lost in the process through heat or mechanical inefficiencies.
However, what we must do—and I'll leave this to better scientists and mathematicians—is compare the effect on the environment of removing motion energy from the ocean and the effect of all the different ways we produce the energy that we're not capturing through tidal turbines—petroleum, coal, natural gas, hydro, wind, solar, etc.
I believe, based on what I understand about the pressing problems of the day, that harnessing the "motion of the ocean" will be less destructive for the planet—and will help mitigate the current disasters of irresponsible energy production—than any production based on fossil fuels.
Nothing is free, unfortunately. All we can do is choose how to pay, and then spend wisely.