This pool of freshwater would need to be many kilometers in diameter to give a high chance of hitting it with an unpowered parachute descent (thanks to unpredictability of the winds). At this point you are talking about doing something that is much, much more expensive than the current approach of a powered landing. How are you going to get so much freshwater out into the sea, anyway? That's way more water than could be carried in the largest cargo ship that exists.

A freshwater lake would seem easier

There are remarkably few freshwater lakes in the Atlantic Ocean.

Lake Michigan seems like a pretty easy target and it's 100% American territory.

The barge was located about 400 miles downrange of the launch site in this case, so if you're landing in Lake Michigan then you're launching from somewhere around western Iowa/Minnesota or eastern Nebraska/South Dakota. When your rocket goes off course or explodes, you'll rain parts and fuel down on Minneapolis or Chicago or whatever other inconvenient population happens to be in the way.

This is a typical engineer conversation.

"Oh, I'm sure I know a lot better than the accumulated centuries' worth of knowledge from actual rocket scientists, and I can come up with solutions to their problems based on no in-depth knowledge of the field and a minute's thought."

Not at all, it was an observation that bodies of fresh water exist that don't require huge effort to make massive floating ponds on the sea.

It was also an observation that this particular body of fresh water is an "easy target," apparently stated without putting any thought into what makes a target easy or hard for this purpose.

Given the lack of fine guidance control that parachutes imply, your "large" plastic liner would have to be miles in diameter.

That's a lot of complexity to add compared to the current landing which boils down at it's simplest to an inverted pendulum balancing problem, hard but the dynamics are pretty well understood. First parachute landings aren't super accurate without a lot of engineering and controls and would rely on calm or no winds down range wherever you're landing to have any chance of hitting the 'pool'. Second that pool would want to float and to my knowledge no one's made anything like that, so they'd have to invent and build then deploy a huge piece of engineering for every single launch.