The Russians have been dumping all kinds of radioactive waste into their oceans.
The Soviets/Russians have never been the eco-friendly sort. They love backing radical “greens” in the west to undermine us, but don’t practice what they preach. The slice of the former East Prussia they own, Kaliningrad, has been their toxic waste dump since 1945. They asked the Germans if they wanted it back, but the Germans said “no thanks.” Probably too polluted for habitation.
I’m not keen on just randomly dumping radioactive waste into the ocean as a standard policy. I have no problems with dumping waste into the oceans if done correctly; send the waste to the bottom of a deep ocean trench in a subduction zone, and let it be reabsorbed into the earth’s mantle.
But for an incident like Fukushima, a controlled dump of radioactive water is OK. Don’t dump it all at once, but do so over about a year’s time, and the world will never notice it.