lifeless ocean

"Lifeless ocean" is an exceedingly strange phrase - the ocean is rarely described as "lifeless" (though the doldrums described in "The Rime of the Ancient Mariner" would be one example). Before this spot of tumult in the caverns, the river "meanders" through a romantic, peaceful sounding paradise, then at this point of tumult the dread begins, the life leaves the water. The fact that the caverns where the river changes are "measureless to man" implies an Edenic original sin problem for Kubla - he has tasted the forbidden fruit, and pays.