it works because there is higher density liquid on one side of a membrane and sea water on the other side... so to use Sea-Pack, you have to also carry along packs of liquid... it seems at some point you may as well be carrying water.

Reminds me of the old joke... "here, this powder is instant water, just add water and mix."

The PUR takes nothing but seawater and muscle power. The Sea-pack requires some other liquid to work... which may as well be water. (although according to the info I read, you are getting some 480 calories from the whole process)