Once you've built the dam, tidal power is free. It produces no greenhouse gases or other waste. It needs no fuel. It produces electricity reliably. Not expensive to maintain.
Tides are totally predictable.
Disadvantages Very expensive to build. Affects a very wide area - the environment is changed for many miles upstream and downstream. Many birds rely on the tide uncovering the mud flats so that they can feed. Only provides power for around 10 hours each day, when the tide is actually moving in or out. There are very few suitable sites for tidal power stations.