IF (big if) the marsh isn't brackish, then I'd use a bandanna to filter out big stuff, then put it through the water filter, then add chlorine dioxide. Or you can do the ClO2 first, but then let the water stand 15-20 minutes before filtering. The filter gets protozoa and most bacteria, but not viruses, which is why I'd double-treat for such an iffy source.

I'd be really worried both about brackish water and concentrations of agricultural or other chemicals. Nemeczek has a good idea about distilling! The extra fuel you'd need is probably lighter than a three day's water supply. I'd experiment at home first, though. As he says, it's very time-consuming.