Welcome to ETS, andrei. You mentioned water for a couple months in your last post. Is that what you're shooting for? Because that really changes the question and is really more a topic for the Long Term forum. Storing water for a family of five for a couple months is a lot of water and is not practical for many/most people. I use three gallons per person per day as a guideline for short-term emergencies. For five people (even if 3 are kids), that's 900 gallons for 60 days at a bare minimum.

Getting back to the canal/marsh--don't forget that conditions during normal times could be completely different than after extensive flooding or after a hurricane storm surge pushes all kinds of crap into the marsh. Flooded sewer tunnels and septic systems, flooded gas stations and factories, even flooded golf courses are going to unload a lot of chemicals and other nastiness that normally don't really get into the marsh. The community pool or lake may not be any different.