Today, while searching “underwater” in the news media, more articles come up about the economic crisis than the environmental one — “underwater” is a term used in the industry to designate homeowners that owe more on their homes than they are worth (the official term is “negative equity”, also referred to as “upside down” mortgages). See for example: “Mortgages: Economy Drowning in Underwater Homeowners.”
Also interesting is that homeowners are more likely to go underwater in bubble states (e.g. California), once the bubble bursts, of course. Two of the OED definitions of bubble:
1. A thin globular (or hemispherical) vesicle of water or other liquid, filled with air or gas.
2. Anything fragile, unsubstantial, empty, or worthless; a deceptive show.
The bubble is a (fragile, unsubstantial) surface of water. These water-related metaphors definitely draw from the negative connotations of underwater spaces — those in which we cannot live, cannot breathe, and are unable to move in the the proper directions (homeowners are stuck with homes they can’t sell). Water is amorphous, unstable, not a solid support, not to be trusted.