Oceans of Other Worlds

The depths of the ocean have always been, and will continue to be, sources of mystery and wonder. But what of aliens oceans? How much more do they provide fodder for storytelling? given that the simple presence of any water on a planet, let alone an entire ocean would be game changing from the life prospective an ocean couldn't fail to tantalize writers. Or what about the concept of an ocean world? A giant sphere of water without a rocky core? What would exist at its center? 
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