Rance
At least some of your questions are answered by the continued operation of the Rance Tidal power station (as mentioned in the article). It operates today, 46 years after construction. Wiki says it cost E94.5 million and took almost 20 years to pay for itself. This style of tidal generator (ie. barrage) is essentially the same as a dam and they can operate for very long times--some say for 120 years; Hoover dam has been generating power for 76 years so far.
It may be different for the tidal flow type turbines as described here but they can access vastly more potential sites with much less environmental concerns.
