Well, there's two questions, right? The first is how harmful tritiated water is to begin with. It's a low-energy beta emitter and, because it mixes quickly and permanently with water, it's eliminated quickly as well. You wouldn't want to go out of your way to drink it, though.

The second question is, what is the environmental impact of dumping it in the ocean? No matter how dangerous it is, the ocean is gigantic. So you want to know: are there environmental processes that concentrate or amplify it?

Later: I had to leave before writing this last part.

Obviously, all things being equal, you'd rather TEPCO keep spending money to keep this stuff out of the ocean. But are all things equal? Is it riskier to try to contain it?