There are times while exploring a tomb when you do want your sim to have the Soaked moodlet – for example, just before trying to cross a fire trap. However, since this moodlet subtracts from your sim’s current happiness value, it is better to avoid it when you don’t need it. So when you need to explore a dive well, a quick change of clothes is in order.
When your sim is dressed in swimwear, exploring a dive well will not give your sim the Soaked moodlet. She will emerge from the dive well fully dried off. Changing to swimwear before a dive is just one quick step, and if you forget, have your sim change clothes after emerging from the dive well and the Soaked moodlet will disappear after she puts on the dry clothes. How those clothes in her exceedingly tiny pocket (while in swimwear) stay dry while she’s underwater in the dive well or where the invisible magic towel comes from and goes to after swimming remains a mystery.