The Schwimmwagen was definitely a joke.  I can't imagine taking one into the Pacific, even a few hundred feet, at a time & in sea conditions not of my choosing ... we're talking about a plan for when an earthquake happens, not necessarily on a bright & sunny day.
It may only be half a joke - pun intended.  An Australian named Ben Carlin took an amphibious jeep completely around the world.  
http://members.iinet.net.au/~daveb/halfsafe/halfsafe.html
One of my favorite quotes is from Burt Rutan, and is along the lines of "if half the people aren't sure you're crazy, you're doing something wrong."   
What about a zodiac and a fold up trailer in a small rented storage shed?  Just thinking of escape here, not something for long term survival by itself.