Oundle, River Nene is a popular wild swimming spot on the nearby River Nene surrounded by verdant river beds. There is a pontoon used by the Rowing Club that makes it easier to enter the river. Upstream of Oundle, Cotterstock and the bridge at Fotheringhay Castle are idyllic spots for a dip. Just off from the Nene, Harpers Brook (on Titchmarsh Nature Reserve) is a lovely swimmable stretch of calm river which comes highly recommended. All between 20-30mins drive from site.
For first time wild swimmers, this piece by Wild Swimming covers the basics of taking the plunge safely.