How do I respond to people saying houses near the beach will be underwater soon?

Absolutely. The storm off the southeast U.S. coast for the last three days did this in North Carolina.

http://www.wilmingtonstar.com/apps/pbcs.dll/article?AID=/20070510/NEWS/70510004/1004/news01

Christoper said it before I could. Beach erosion is absolutely no joke. It is a serious threat to the seaside homeowner.