you may have seen the Sea-Pack written up in Blue Water Sailing or Latitudes and Attitudes but I wanted to pass it along for those who haven't seen it because quite frankly I think is pretty dang kewl. It seems like the perfect thing to stuff in your life raft case when its being serviced or to carry in your ditch bag.
Here is a very technical description of forward osmosis, I think of it as water moving through a membrane from an area of HIGH water concentration to an area of LOW water concentration, and as this happens the membrane seperating the two areas filters everything else out. That's probably not 100% accurate, but I think its a good lay description.
The packs can last from 3-5 years unopened which I think is very reasonable shelf life for safety gear like this. I think it would make sense to by one while your still coastal cruising (and can be saved quickly) and then buy another one each year until you have 3 and then you can rotate replacing them only having to buy one each year instead of replacing all of them all at once like you do with your flares.
Anyway, I think its a great product and it seems like something I will have aboard my vessel.