Nuclear powered ships cause a lot less harm to the environment than diesel, fuel oil, or bunker fuel powered ships. If those ships are destroyed, they release enormous quantities of toxic oil into the environment, where it persists for decades.
The thing about nuclear ships is, water is an absolutely awesome radiation barrier.
See this spent fuel pool, with extremely radioactive fuel rods in it?
You can go swimming in it.
Water is that good at blocking radiation.
Nuclear powered submarines have sunk. You know what happened?
You know what happens when diesel electric subs sink?
Widespread ecological disaster.