Define "know". Do we have it carved in tablets of stone from on high? No. Are we reasonably confident? Yes. There are already hotter things in the ocean (e.g. black smokers). What kind of effects are you worried about?
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".
