Being underwater means that the debt on a house is more than the value of the house (i.e. negative equity), and when it's "serious" means it more than 25%. Apparently there are over 9 million homes in the US that are still seriously underwater, and fewer distressed properties overall.
However, still 45% of properties in foreclosure are seriously underwater, so there are still some great buying opportunities in the States for investors who are looking for an international property portfolio.
At The Investor Hub, we are connected with real estate professsionals in the US that seek out great buying opportunities, so get in touch if you want to start a conversation around that.