You might also want to consider the new forward osmosis water filters from HTI. They now have a version that will filter seawater, and their entire line to me seems to be the easiest filter system to pack and use. I haven't actually ordered from them yet, but (unless anybody here has negative experience with them) I'm planning on getting their camelback style filter for my BOB.

http://www.htiwater.com/hti.html
http://www.htiwater.com/seapack.html?Vl=5&Tp=2