Is it a big deal if they break/leak? Are there any theoretical problems that I, as not-an-oceanic non-nuclear-waste expert, am not aware of? Not just over 5 or 10 years, but over hundreds of years? Are we able to monitor them for these problems? How difficult or expensive is it to fix any problems?

I ask because I genuinely don't know. I'd be OK with it if scientific consensus and consultation between the ocean and nuke people said we know enough about them to say there aren't issues. I don't want to just say "lets just dump them in a place I don't personally care about that's really far from me".