Might make more sense to put the RTG in the nosecone; why ship something radioactive and useful all that way just to throw it away?
Oh great, let's hose up the environment on our first visit...we will end up doing this anyway, but radioactive chuncks on the sea floor?
It's probably substantially less damage than the rocket required to launch the extra mass that would be required if we used an alternative, heavier, method.
Just make sure that we drop it near a black smoker and accelerate evolution for the Europans. I doubt there is much background radiation on the bottom to stimulate random mutations.
Water is so good at blocking radiation, that an RTG dropped into that ocean would be absolutely nothing. It would be dangerous to anything within 5 meters of it, which again - is absolutely nothing.