Obvious question is, now did Jellyfish get into the water? Are they drawing salt water for cooling? As far as I know, jellyfish cannot live in fresh water.
1/3 of all water, uninhabitable.
Nearly every nuclear plant on a coastline is salt water cooled on the secondary heat exchangers. There is usually a freshwater loop between the ultimate heat sink of the ocean and the comdensers for turbine steam. It’s not a hard engineering issue to do liquid to liquid heat exchangers. Every Navy sub has them, aircraft carrier too.
Backflow doesn’t work at high flow rates since the capacity of the cooling loop is sized to the full output load plus a percentage margin. You would need to double the capacity to pull half off line to backflush the other half. I guess you could do 4 or 8 parallel lines and oversize by 25 or 12.5 percent so you could take 1/4 or 1/8 offline to backflush either way it a continuous OPEX that would only be needed very infrequently.