It's mostly speculation, but the reasoning is pretty sound: if Pluto's rocky core has a certain level of potassium, "its decay could produce enough heat to melt some of the overlaying ice," says New Scientist. The assumption is that Pluto does, since the Earth has 10 times that amount despite being closer to the sun and therefore likely having much less potassium in its core than Pluto. We just hope having an ocean makes Pluto feel better about not being a planet anymore. 

[New Scientist]