There is a sort of lake on the north east side of town. There used to be a boat that would give people rides around the lake. I would imagine people used to swim there too, but because of all the flooding that has occurred here over the years, any old coins or jewelry would probably be 2 feet deep or more.
There could be other locations along the river where people used to swim, but I am not aware of them.
Old post cards and pictures can be a wealth of information for finding old sites, but unfortunately, around here, so many that sound good turn out to be built on, dug up, or filled in. I have seen pictures of several old picnic groves from around the turn of the century that show people having a good time. Most of these sites have been near a stream and most of these streams flood.
I will still check out these places, but overall I have done poorly in such a location.