It would be safe and effective if we had a mechanism of distributing nuclear waste evenly throughout the entire volume of the Earth's oceans. But we don't have that and that's impossible to build!
Dumping nuclear waste into the ocean will, mathematically, result in a globally acceptable concentration of nuclear waste in ocean water - but it will be locally problematic for wherever we put it, and that's assuming it stays put.
Dumping nuclear waste into the ocean will, mathematically, result in a globally acceptable concentration of nuclear waste in ocean water - but it will be locally problematic for wherever we put it, and that's assuming it stays put.