In The Sims and The Sims 2, Sims who were very outgoing would be naked when using a hot tub, at least when kids and teens weren't around. However, true skinny-dipping did not enter the world of The Sims until The Sims 3: Late Night.
In The Sims Bustin' Out skinny dipping is something sims do in a hot tub which is often the result of being very outgoing, though sometimes the sims will do it even without being outgoing. The player has no control over whether their sims skinny-dip.
Skinny-dipping can be performed in hot tubs and pools. Any number of sims can skinny dip and they will leave their clothes on the edge of the pool or hot tub.
Sims with the kleptomaniac trait in The Sims 3 can steal clothes from Sims who are skinny-dipping. Those Sims who have had their clothes stolen will have to walk around in a towel until they can access a dresser, or at least a shower. Celebrities should perform this action carefully, as paparazzi might photograph them.
Note: Young adult and adult Sims cannot skinny dip while children and teens are on the lot; they will automatically leave the hot tub when a child or teen enters the lot. Likewise, children and teens cannot skinny dip.