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.

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.

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.