First your sim must have the party animal trait.
Then click on a pool or hot tub and click on skinny dipping.
My sim had an option to ask someone to go skinnydipping with them under the Romantic option.
simschristy12 wrote: »
You need to own the expansion pack 'Get Together' to be able to have the option to Skinny Dip in a pool or hot tub. So if you don't own the 'Get Together' expansion pack, you won't have the option to Skinny Dip.