I thought that if it came to an emergency they could keep the reactors running, only the exhaust water would be too hot and river wildlife would die around the vents.

France has already upped the maximum temperature legally allowed two or so years ago. And while short term that is a solution, the fact is that us humans need working ecosystems to survive. It's not just fishers being impacted, ecosystems are connected and we're cutting into them at record pace anyway.

So instead of running hot, the river runs dry?

Any thermal electricity plant uses an insane amount of water. For every kWh of electricity generated, about 1.5-2 kWh need to be cooled away.

A pure air cooling system is theoretically possible, but it drastically reduces the plants efficency, as the energy you can recoup at the turbine is directly dependant on the temperature difference between the hot and the cold side.

So in any way thermal plants are never going to be an option that is favourable to build now, over building renewables, except for a small degree of net stability that can be provided by already existing plants.