Is there anywhere near central Osaka where I can swim for free?

I was living in Osaka a few years ago, so I'm not sure if it's still valid, but at least back then in Sakuranomiya park there was a nice beach on the river. The beach is located approximately here. It is fairly close to central Osaka, so it should fulfill all the requirements.

There are none. I did a throughout Google search for free swimming pools in Osaka (?? ?????), and, as I have expected, there turns out to be none. There are a few cheap public pools which only charge a few hundred yen, though.

Osaka does have a harbor, but there are no beaches open for swimming within city limits. It would be likely illegal and surely hazardous to jump into the sea, since (1) the water is dirty and polluted and (2) Osaka harbor is very busy and you don't want to run into cargo ships. Same for the rivers: you aren't allowed to swim in them plus they're dirty. So, I guess that leaves you with no options here.