Could it be this easy?

Not available for purchase yet, but Puri has submitted it to the 2013 IDEA Design Awards competition.

Its a water bottle that can purify fresh water and desalinate sea water.  Just put in some contaminated/salt water, and pump.  Its a RO desalinator in a water bottle!

If/when this makes it to the market, this will end up aboard many salt water-based boats, and in every ditch bag and life raft.

(And a tip of the hat to Boat Projects, where I first saw this.)