I always wondered about another method which would be to use the actual Tidal force to generate electricty. A quick google search turned this up.
http://www.alternative-energy-news.i...o/tidal-power/
I think you could even make it better my having sequential turbines so as to give a bost effect from the first to the second. Like the do in Aircraft engines.
However in either method one must closely examine the impact on environemnet. while out of sight is good for human beings, it may not be so great for the wildlife out at sea.
As to the question of corrosion, what material is used in submarines and ships to stop the same corrosion. Could not such material be used in construction and design of these turbnines.