According to Official Guinness Records,
The largest skinny dip was achieved by 2,505 women, during an event organised by Deirdre Featherstone (Ireland), in Wicklow, Ireland, on 9 June 2018.The event was organised to raise awareness on breast cancer and to raise funds for Aoibheann’s Pink Tie, a National Children’s Cancer Charity: http://www.aoibheannspinktie.ie/.
For a complete list of 2018 records, please visit 2018 Guinness Records in Ireland.