If I recall correctly, it wasn't a beach until after the late 70's when some idiot decided that dredging the Cherwell was a good idea to prevent flooding.....Made a right mess, ruined the water course and made zero practical difference to flooding.
Took years to recover and to be honest, it never really has. I don't get down there much these days, but when I have gone for a walk, the typical water level seems to be a bit lower than it once was.
But yes we used to go swimming at Somerton, Upper Heyford, Lower Heyford and near to Steeple Aston. Where we mostly used to swim in Lower Heyford, the river bed was gravel that had accumulated naturally and there was a shallow end for kids (about knee deep) with a deeper section (6-7ft) on the bend. The river bank was about 4ft above the water, so a running jump or dive into the deep-end was a lot of fun. Somerton was similar. When the river was dredged, this was completely ruined, the flow of the water changed and it ended up all mud.
In the 60's to pre-dredging 70's, pretty much all of the young families in the villages would swim in the river at the weekend. Most of us used to swim throughout the summer months in the evening too when the weather was goo. There was also a spot about halfway between Upper and Lower Heyford where the water was about 10ft deep. We used to jump out of trees into the water and built a diving board one year. That got ruined when the local water authority decided to put a sewage outfall into it.
In my Mum's youth, the canalboats were horse-drawn, so she used to swim in the canal at Upper Heyford. Now of course the canal is too muddy for swimming. We used to canoe along it though, pretty much from Somerton Deep-Lock down to Dashwood's Lock and sometimes a bit futher south.