Great question. The answer is: it largely depends on where the turbines are sited. Obviously, if done irresponsibly, a turbine field on the ocean floor could have disastrous results for an existing ecosystem—and therefore proper siting must be a priority for all tidal power companies. However, often in areas of strong tide, the power of the current is such that ecosystems are unable to develop—leaving only the water rushing over bare rock. In these places, a turbine field would have minimal impact.
Here's a good answer I came across:
"Tidal energy is a renewable source of electricity which does not result in the emission of gases responsible for global warming or acid rain associated with fossil fuel generated electricity. Use of tidal energy could also decrease the need for nuclear power, with its associated radiation risks. Changing tidal flows by damming a bay or estuary could, however, result in negative impacts on aquatic and shoreline ecosystems, as well as navigation and recreation.
The few studies that have been undertaken to date to identify the environmental impacts of a tidal power scheme have determined that each specific site is different and the impacts depend greatly upon local geography. Local tides changed only slightly due to the La Rance barrage, and the environmental impact has been negligible, but this may not be the case for all other sites. It has been estimated that in the Bay of Fundy, tidal power plants could decrease local tides by 15 cm. This does not seem like much when one considers that natural variations such as winds can change the level of the tides by several metres."