Even if the waste starts to leak, it will be just local problem and not big catastrophe. Compared to global warming, burying nuclear waste is more responsible solution for the future generations sake.
[0] https://en.wikipedia.org/wiki/Ocean_disposal_of_radioactive_...
i'm sure that its safe to dump small amounts of radioactive material into the oceans, but if you start to build more reactors and dispose of all the waste into the ocean, the dilution will soon cease to be enough to offset a global ... disaster.