In The Sims and The Sims 2, Sims who are very outgoing would be naked when using a hot tub, at least when kids and teens aren't around. However, true skinny-dipping did not enter the world of Sims, until The Sims 3: Late Night.
In The Sims Bustin' Out, skinny dipping is something Sims do in a hot tub. While it is often the result of being very outgoing, sometimes the Sims will do it even if they are not outgoing. The player has no control over whether Sims skinny-dip.
The Sims 3
Any Sim 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.
Young adult and adult Sims cannot skinny dip while children and teens are on the lot regardless if they are the player's Sims or not and whatever the Sim is in a different room; they will automatically leave the hot tub when a child or teen enters the lot. Likewise, children and teens cannot skinny dip.