When I was 14 I lived on houseboat on canal in Essex near Heybridge.  Next to the canal was a large gravel pit in which there were fresh water lakes.  As I used to get a rash from salt water I tried swimming in the lake nearest to our mooring one day.  I had a pair of shorts on, not proper trunks and after a while I dived in and the shorts came off - I had to search around for them and found the sensation of swimming naked so pleasant that from then on I swam naked whenever I could.  I became a 'proper' nudist when in Germany in the forces.