By this do you mean using the tidal currents to drive turbines that will then generate electricity? If so, then I can see one very big problem. The turbines would be murder on the fish populations in the bays where they are set up. As the tides sweep in and out through the turbines, they would carry the fish that live at those levels through the turbines as well. Can you say sushi, boys and girls? The effect would be the elimination of fish at the depth and in the area of the tidal turbines. This could put a lot of stress on the entire ecosystem of the area.

Erosion (but the true global effects are minimal), collecting trash in a giant mass off of California (but the trash part is man made). Um. . hmmm. . Nothing that doesn't effect humans that humans have not brought upon themselves.

"Environmental impact The placement of a barrage into an estuary has a considerable effect on the water inside the basin and on the fish. A tidal current turbine will have a much lower impact. Turbidity Turbidity (the amount of matter in suspension in the water) decreases as a result of smaller volume of water being exchanged between the basin and the sea. This lets light from the Sun to penetrate the water further, improving conditions for the phytoplankton. The changes propagate up the food chain, causing a general change in the ecosystem. Salinity As a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem. "Tidal Lagoons" do not suffer from this problem. Sediment movements Estuaries often have high volume of sediments moving through them, from the rivers to the sea. The introduction of a barrage into an estuary may result in sediment accumulation within the barrage, affecting the ecosystem and also the operation of the barrage. Pollutants Again, as a result of reduced volume, the pollutants accumulating in the basin may be less efficiently dispersed, so their concentrations may increase. For biodegradable pollutants, such as sewage, an increase in concentration is likely to lead to increased bacteria growth in the basin, having impacts on the health of the human community and the ecosystem. Fish Fish may move through sluices safely, but when these are closed, fish will seek out turbines and attempt to swim through them. Also, some fish will be unable to escape the water speed near a turbine and will be sucked through. Even with the most fish-friendly turbine design, fish mortality per pass is approximately 15% (from pressure drop, contact with blades, cavitation, etc.). This can be acceptable for a spawning run, but is devastating for local fish who pass in and out of the basin on a daily basis. Alternative passage technologies (fish ladders, fish lifts, etc.) have so far failed to solve this problem for tidal barrages, either offering extremely expensive solutions, or ones which are used by a small fraction of fish only. Research in sonic guidance of fish is ongoing." "Energy efficiency Tidal energy has an efficiency of 80% in converting the potential energy of the water into electricity,which is efficient compared to other energy resources such as solar power or fossil fuel power plants. Global environmental impact A tidal power scheme is a long-term source of electricity. A proposal for the Severn Barrage, if built, has been projected to save 18 million tons of coal per year of operation. This decreases the output of greenhouse gases into the atmosphere. If fossil fuel resource is likely to decline during the 21st, as predicted by Hubbert peak theory, tidal power is one of the alternative source of energy that will need to be developed to satisfy the human demand for energy." Source and further information: http://en.wikipedia.org/wiki/Tidal_power

There is minimal research in this area of the effects of tidal energy development on marine mammal, fishes, and organisms. At this point, much of the perception of potential wildlife effects stem from the major bird mortalities in huge windfarms. There are similarities (tidal blade tips or turbine type desings can spin 25-40 mph ( speed at blade tips) Many programs that are gearing up for this are in pilot phases and still working out the best technologies for the varying conditions. The discipline is about 15 - 20 yrs behind wind power development - that is, although there is fine engineering going into the tidal and wave energy systems, there are physics and materials issues still being worked out to establish tidal power as a mainstream alternative energy. I am concerned about many environmental effects involving marine life and physical processes, and it is yet to be determined if the devices are creating these flow vortexes that guide fish and mammals into them and harm them. Fish and mammals generally have avoidance behaviors but those could be altered given turbid conditions, noisy conditions that cause abnormal acoustic situations for fish, or camouflaged conditions of the devices as biofouling occurs . Many organisms that are drift-type such as plankton, larvae types, jellyfish, salps, or other slow moving/responding swimmers may be most affected on a regular basis. Although there is conjecture about the array of impacts that are possible, the best basis for judging the negative effects would come from extensive modeling of the affected processes. Underwater cameras to evaluate the habitat use of the proposed area ahould also be required. Other factors to consider are the shoreline and bedline modifications that occur from laying cables and shoreline development to handle the power generation.

is that even a sentence?

There are a number of answers to your question here - http://www.answerbag.com/q_view/352789

poor fishies!!