The ocean wouldn't be the worst outcome. The core material is all forms of metal so it would sink. We presently use large "swimming pools" of water to insulate radioactive material right now so a radiation source at the bottom of the ocean would not pose a lot of immediate threat. It would leave some residual radiation in the water though so you would want to get down there and pick up the material at some point. Upside is it would cool down and solidify very quickly.