While intuitively dumping nuclear waste on the ocean seems like a terrible idea, if one actually does a cost benefit analysis of dumping glassed and cannistered nuclear waste on the abyssal plane it becomes tricky to find downsides.

The ocean already contains a billion tons of uranium and is mildly radioactive, the cannister and components are far too heavy to float or get carried much by currents, water is a fantastic radiation blocker (a few meters away and you're quite safe) and carries away exces heat and there's almost no life on the abyssal planes.

I really should get around to properly writing up all the details and background knowledge in a blog post at some point... (the above is missing a lot, including references, common objections and numbers)