Obviously an old warning.
Not really a swimming hole, but a river. I just suddenly have the Andy Griffith theme stuck in my head. Hmmm. Anyway, the sign is warning people to be careful of swimming in the river. But the sign doesn't seem to have been tended for sometime.
This area makes me curious as hell actually. The grass here is mowed once or twice a year when it gets too high, and there is a lot of paved areas closer to the river, yet otherwise the area is deserted with all roads going into it barred by chains with signs saying "forbidden". Part of me thinks the city once poured money into the area† but their plans fell apart and now they just keep it locked up and only do the minimum of care to keep it from becoming entirely overgrown.
But I don't know, nor can I find anyone to talk to who does know. Very odd.
Footnotes
†: Perhaps in the golden 80s, the height of the Japanese bubble economy, when everyone was so rich money didn't matter and cities were throwing it away on stupid projects.