>Is it the turbines themselves which are very difficult to maintain in salt water?
The real key is they are far more difficult to maintain than wind turbines. Also, dams are ecological nightmares and disrupting huge swaths of coastline is not ideal. Dams also made use of very specific geological conditions where a maximum amount of water power could be achieved with the least amount of money, time, materials, etc. Coastlines don’t have steep cannon walls to build up hundreds of meters of water to push through turbines at high pressure. Coastal tides are moving water meters high, not hundreds of meters. It isn’t that it can’t be done, it is just that it can’t be done more economically than other technologies.
The real key is they are far more difficult to maintain than wind turbines. Also, dams are ecological nightmares and disrupting huge swaths of coastline is not ideal. Dams also made use of very specific geological conditions where a maximum amount of water power could be achieved with the least amount of money, time, materials, etc. Coastlines don’t have steep cannon walls to build up hundreds of meters of water to push through turbines at high pressure. Coastal tides are moving water meters high, not hundreds of meters. It isn’t that it can’t be done, it is just that it can’t be done more economically than other technologies.