@blackle This can be a wild card because water in a lot of cases acts as a neutron moderator that allows nuclear fission to happen. Heavy water(water with a heavy hydrogen isotope) is a popular neutron moderator and naturally occurs in oceans.

Neat thing is that they discovered a natural nuclear reactor that functioned millions of years ago in Oklo Mine in Gabon. The uranium deposits regularly underwent a nuclear reaction every time they were exposed to rising ground water levels.

(there's even theories that a natural nuclear reactor might have been responsible for the moon being born from the earth)