Yes. We've dumped tens of thousands of tons of nuclear waste into the ocean, with no observed ill effects. This is very strong evidence that another 900 tons will have no effect either.
I don't think that's how it works. For all we know, in ten years, parts of the Atlantic Ocean could become uninhabitable by marine life due to radioactive plumes. Or not. The history of radioactive waste disposal is very short. Dumping further could exacerbate future problems. That's kind of why we banned it.
Personally I think deep bore hole disposal in geologically inactive regions makes the most sense.
The areas around the nuclear waste disposal sites are monitored for radioactivity. Radiation drops to ambient levels very sharply. Radioactive material decays over time - it'll get strictly less radioactive as time goes on.
Radiation levels at such sites generally drops due to dilution. As we have seen in the case of other wastes like mercury and dioxins, you can get bioaccumulation of this waste in the food chain causing environmental damage.
