RE: Prepping my bug-out cave for the possible coming collapse of civilization
For water, have you tried boiling seawater, and collecting the condensation? You don't want to be depending on bottled water...
Another option is to haul out 5 or 10 NEW plastic 55 gallon trash containers and a package of bags to line them. The bags can be reused, but will make keeping the water clean from algae easier long term. Fill the barrels either from the creeks or by collecting rainwater via a tarp, letting it run off directly into the barrels. Put a sheet or something over it to avoid drowning small animals or collecting debris. Water passes through, but the things mentioned don't usually get in. Except dirt! Let it go to the bottom. And maybe use that part for housecleaning after a rough filtering?
You can also transfer the water from the barrels to NEW plastic gas or water containers. The gas type is usually a lot cheaper, and comes in different sizes.