'The meaning of the "one clear call" seems straightforward--the call to return to the sea, or metaphorically, to go beyond the edge of this mortal coil. '

To go back to where we started: it's my contention that the 'clear call' refers literally to some specific call that would be a familiar feature of sea-travel, such as the 'All aboard!' announcement referenced above. Perhaps that's what you meant? Or do you mean more of a vague 'I must go down to the sea again' sentiment, in which case, the 'call' is already metaphorical ... ?