Those countertop RO systems are for getting junk out of tap water or well water. They're not intended for sea water and sea water RO machines are a lot more expensive and need much more energy. They do have some on that web site and places like West Marine have them too.

http://www.freedrinkingwater.com/seawater-commercial-detail.htm

Katadyn makes some portable units but again, the practical ones are pretty expensive and need electric power.