There's already ~4 billion metric tons of uranium in seawater. Dissolving the entire reactor and dumping it into the ocean would have no measurable effect. RTGs are actually more dangerous, as they require isotopes with high enough levels of radioactivity to generate useful amounts of heat, and those materials are at their highest levels of activity the moment they are produced...you can't just delay turning them on until after they've safely launched.

Thorium is just another possible fission fuel. It's often proposed to be used in a molten salt reactor, but molten salt reactors are not all thorium reactors and thorium reactors are not all molten salt reactors. There's no shortage of safe uranium reactor designs, it's just been impossible to get them implemented because of the anti-nuclear groups.