Radioactivity occurs naturally in the sea and everywhere in nature so this could make sense. I guess it's a matter of how fast it will be released and the levels implicated. Why not have one or several independent entities supervise the release? Even environmentalists could be part of that supervision.
It might be better to dissipate it than to keep it all concentrated in one spot. The more transparent the process will be the better. If they keep everything hidden behind closed doors than I guess it gives a good reason for everybody to be suspicious.
That radioactivity came from nature in the first place by concentrating it so, it is just a matter of returning to nature in a safe way, unless there is solid particles it the water which they say have been filtered out. It's not like dumping used radioactive fuel rods into the ocean.