Cars are also not safe, especially at 200+ km/h but somehow it’s OK to drive them this fast in Germany.

Edit: What I want to say is that there is no absolute safety.

Waste from nuclear weapons is not the same as waste from commercial nuclear power plants.

Unfortunately use of fossil fuels also continues to hit record numbers year after year.

You basically need a few conditions to be met to make this useable: tide needs to be high enough, there needs to be suitable geological formation that enables building of such power plants, it has to be publicly acceptable to build there, and you need to connect it to the grid. The last two can especially cancel eachother out.

However, this assumes you use potential energy. What you are envisioning might be more like current power (so kinetic energy) where I’m not sure what the limitations are. Perhaps it’s not too practical to build huge plants underwater in locations with relatively constant current and connect them to the grid