I used to work in this area for a short time, so I have some insight.
The company made what the newspapers called a "fish disc", basically flashing lights and speakers placed underwater to drive fish away. There were also UV lights to stop stuff growing on them. They worked but were problematic. Difficult to install and in need of a lot of maintenance. Not 100% effective either, the nuclear plants they were protecting still needed filters and regular cleaning, just less of it. And of course when one was hit by lightning and knocked out temporarily, the plant was in trouble.
Fighting nature tends to be a losing battle. Much better to try to integrate with it. If you are building say an offshore wind turbine, you just need to accept that stuff is going to live on and around it, and design it to deal with that. Fortunately it's not that hard with a stationary turbine, but for a nuclear plant that relies on fresh water for cooling, dealing with stuff living in the water and the water itself heating up is a major problem.
Oh yeah, some of those French plants shut down because the water got too warm in the heatwave.