Diving sims seem to reset all sims on the diving lot to the nearest "ground" when your sim tries to explore a cave. However this does not always happen, my guys is that it happens because it needs to load a "hidden island" that you found.
Diving Skill: 5
Mods: Yes, but not the cause of the problem because the issue also occurred many times during the dutch preview when there were no mods installed
Diving lot: the second lot you unlock (Kluis van Davy Jones in Dutch)
Will keep you updated if I find out more, the low skill could be the issue
Edit: Also happens on the first unlocked diving lot (Rotsenrif in Dutch)