For no apparent reason (this is a song, not a story, so I'm pretty economical with exposition) nearly every tetrapod on Earth has vanished, and water is gone from the oceans. Just sort of hand-waved out of existence; too many clouds if it all evaporated.
The viewpoint character, the only remaining tetrapod we know about, is sitting on the lawn of a hotel in coastal New England looking out to sea, or what used to be the sea. There's nothing but mud. The water went away, leaving fish, whales, a thousand thousand slimy things, etc. (What? Whales are tetrapods? Look, a guitar solo!)
It's been six or eight weeks since what I'll call The Event. What would a former harbor smell like? I don't want to dwell on anything ghoulish or disgusting, but it's a key part of the scene.