Tidal power, AKA hydrokinetic power, is indeed looking more and more attractive.  Right now there are multiple sites along the US coast where rotary or helical turbines are being proposed for testing, under permits from the Federal Energy Regulatory Agency (FERC).  While the technology is still in its infancy and impacts on marine resources are largely unknown, hydrokinetic turbines are likely to be much more friendly than are barrages.

Some potential advantages of hydrokinetics:Predictability.  Unlike winds, tides occur with great regularity.  Both the magnitude and timing of tidal cycles are known far in advance, making it possible for a utility to know how much power will be generated and what other sources should be used to meet demand.  While Umbra rightly points out that periods of slack water between tides will result in no power, we Grist readers know by now that our future energy policy will have to rely on many eggs in many baskets.  Tidal energy could certainly be one of those eggs.  If it ever becomes economically feasible to store power, e.g. by using electrolysis in hydrogen cells, tidal energy could power communities around the clock.  Also, because tides peak at various locations along a given coastline at various times, by placing turbines in different locations it may be possible to level power production regionally.Location.  Much of our population is concentrated along our coasts.  Where tidal energy potential is also high, placing turbines just offshore could result in vastly shorter transmission lines than we see elsewhere.  Again, by placing smaller eggs in nearby baskets, we are eliminating the total waste of electricity (as waste heat) that currently results from long-distance power transmission.Scalability.  In order to generate power from rivers, we typically build dams, which are similar to tidal barrages in their environmental impacts.  Dams and barrages block fish migration and substantially alter flow regimes, sedimentation patterns, and water quality.  In contrast, a hydrokinetic turbine will only affect its immediate environs unless many are installed across a narrow waterway.  In case of unforeseen envvironmental problems the turbines are easily uninstalled and relocatable.

For more information on hydrokinetics, including maps of North American tidal energy potential, a description of turbines generating power in the East River off Rooseveldt Is. in NY, and another in British Columbia, see this website:

http://www.epri.com/oceanenergy/

For papers presented at the January 2007 Ocean Energy conference here in Alaska, see:

ftp://ftp.aidea.org/Alternative%20Energy%20and%20Energy ...