Wetsuit
The wetsuit is an item in Keen 4. Keen needs it to be able to enter the Three-Tooth Lake and especially the Well of Wishes, which is an underwater level. He can find the suit in Miragia. The level is finished by collecting the suit.
Princess Lindsey hints to Keen that "There's gear to help you swim in Three-Tooth Lake. It's hidden in Miragia."
It is not clear where the wetsuit comes from, but it is very likely that it was created by the native Gnosticenes of the planet.
Modding info
Technically speaking, Keen does not need the wetsuit to enter Three-Tooth Lake if he accesses it without going through the shoreline, i.e. by using the no-clipping cheat and walking around the entrance to the lake. In the same way, if you use this method, you will not need the wetsuit to enter the Well of Wishes, as there is no code in the game engine that checks for the wetsuit before allowing you to enter the level, regardless of whether you're using the level warp cheat or not. This is useful for modders to know, as missing this item will not stop the player from entering the level; rather an obstruction must be placed on the world map that can only be passed when the player is in possession of the item.