Mr. Ritter did a whole article on this subject, search the main webpage and find it. He recommends and I agree on the PUR Survivor with a leg strap. There are bigger ones if weight/space are not an issue.

You can find these on Ebay very cheap considering the retail at about $800 +/-. Just search PUR reverse osmosis.

If push comes to shove boiling the water and collecting the steam or just leaving it in the sun, will produce drinkable water and edible salt. The sky is the limit and the imagination could run wild with ideas on how to collect the fresh water. I know a desert survival trick is to put a tarp over a hole with a pebble in the middle (making and upside down cone) and putting a container under the point of this cone, something like this could be done over a container/puddle of seawater with the fresh water catcher floating on the surface of the seawater.