I may not be the most clever bear, but when looking through the myths and religions, I like to find logical explanations for seeming miracles. Sometimes, as in the case of the Reptilian Gods, the theory isn't warm and welcoming and in others there may indeed by a scientific explanation that has been either lost or suppressed.
One such question is regarding the mermaids Atargatis and Melusine. Plus we have the legend of Venus rising out of the sea, or Jonah and the whale. I'm leaning towards the advanced civilizations having submarines and wet suits, but I could be wrong. It may not be as romantic, but perhaps I will be able to put it in my books without ruining the sense of mythical legend that these figures are currently endowed with. :-)