That 200 gpd is fresh water and probably won't deliver it's rating. Membranes are tested in relative warm water, I think about 77°F. And the 200gpd is 24 hour day.
It will work in a river or fw bay, but you'll need more filters and you'll need a uv light or some final sanitizing. I wouldn't want to drink the water of many rivers no matter how many filters.
I built my own, but probably spent more than buying
one of the self installed, modular sets sold on Ebay and some people here. Still a few thousand $, but parts
and filters are standard and available from many sources. I also got a lot of bad advice.
You really want one that runs a short time daily. I run mine about an hour a day when charging