For example radioactive cesium (one of the elements they are dumping) can damage cells in the body and cause vomiting and a whole lot of other problems. It also ignites in the air.
With things like this, the level of concentration is important. The ocean is huge. Like, really huge.
We're bombarded with radiation at all times (less so if you live deep underground). This is very different than being in a fallout area after a nuclear bomb has gone off - with the key reason being the amount of radiation, that is the concentration of radioactive particles in the area.
Also, I don't think I'd worry too much about the element catching on fire. The diffusion will likely take care of that aspect, and there'd need to be some sort of catalyst to free the molecules from the water.
Again, I'm not qualified to opine. I'm merely offering some information that I do know about.
I do know that we were able to measure higher radioactivity in the ocean after the tsunami that took out the power plant in the first place. It was still within 'healthy' limits, but there was an increase - and that's from clear across the Pacific Ocean.