It is interesting how many people first think of volcanoes when talking about warming ocean waters. Wouldn't the amount of volcanic activity required to warm ocean waters over the vast expanse of any ocean dwarf anything that has ever happened, certainly in recorded history? Also, if most of the heat in the interior of the Earth comes from natural decay of radioisotopes, if anything, shouldn't the volcanic activity be diminishing ever so slowly? The Earth after all has probably less than a third the radioisotope load it did when it formed.