Interesting concept, but it looks to me to be too complicated to be practical. It would surely add both cost and weight to the weight and cost of a standard raft.

Small desalination pumps are available for about $700 and they are light. And pre-packaged water in foil pouches are cheaper still and not all that heavy. You could also give solar stills a shot.

It isn't a terrible idea, and I don't wish to discourage designers, I think they need to keep at it, but start over.