Not everyone in the area wants to swim in the sea. Here is a list of inland locations, with varying amount of detail on the pages linked.
More locations can be added, if I hear of them and if appropriate.
Three Holes, Hilgay, Ouse Bridge, https://www.imogensriverswims.co.uk/blog/places/locations-for-s-lincs-n-fens-rutland-swims-12-15-july/
Nar river locations: Pentney Abbey, Narborough and between there and West Acre, West Acre, South Acre, Castle Acre
Leziate lake, club operates on west side (no facilities or safety cover), membership obtainable on one day of the year in March. Informal swimming occurs on the south side.
Pentney lakes (organised venue), https://kltri.co.uk/training-overview/swim
Area included: see map below, and interactive map showing borders of King’s Lynn and West Norfolk District, https://maps.norfolk.gov.uk/highways/maps/district.aspx
Please note: all swims or paddles are at our own risk. Including a location does not indicate that it is recommended or that it is safe; each must do their own risk assessment each time they swim or travel to a location. I accept no liability for the choices that people make. There is Swim Safety info on this website, and on Outdoor Swimming Society website, Survive section. Please follow the Outdoor Swimmers Code.