The ocean is big and can absorb a lot of waste, diluting it and making it "safe".

The USA, Japan and scientific authorities have been at pain to explain how releasing Fukushima water into the Ocean is "safe".

They are probably right to some extent. The ocean will dilute the Waste in this one instance. 

It's the same logic as people can poo into the ocean. The ocean due to vastness, still remains "clean".

The problem will come when everyone think it's ok and start dumping waste into ocean.