The only downside is the “waste” is quite valuable. MSRs can also use it, and their waste is only dangerous for ~300 years.
The waste, likely packaged in torpedo shaped structures made of corrosion proof, high density, high strength materials, would arrive at somewhere around 150 MPH. Being extremely dense and heavy, they would penetrate deeply into the sea bottom, estimated at 35-55 meters deep. There’s no chance they’d somehow be forced back to the surface. Even if they were, being sealed they would be harmless.
Even if by some miracle one broke open at the sea floor, the nuclear material is dense and wouldn’t disperse. There’s very little life to affect down there regardless.
For a thorough analysis (which shows how useful the frontier models are getting at this kind of thing), see https://x.com/i/grok/share/276607c72b5e4f639527935cbc614fa0