This is what I found just from a cursory search:
Slowly releasing Fukushima tritium-contaminated water into the Pacific Ocean is a very good idea. No other radioactive elements are in the water and tritium just isn’t very radioactive - it self-dilutes in water really quickly, even in our bodies, making it impossible to harm anyone or anything.
www.forbes.com
More than a million tonnes of contaminated water lies in storage but power company says it will run out of space by 2022
www.theguardian.com
Dumping contaminated water into the ocean could damage Japan's local fishing economy. Contamination could also travel to South Korea's coast.
www.businessinsider.com
So, it's still controversial, and there are both positives and negatives about it. Can't see that anyone else is contaminating sea water with radioactive material, as Fukushima seems the only one in headlines.
Concerning oceans mixing, here is a map of ocean currents:
So, I can't see how oceans can't mix. Where is that info coming from? Let's see the source.
TD