Yup, just happened to me.  Syndicate NPCs could also dive in and drown. I can't seem to recreate that anymore maybe they fixed the NPCs. Now it's the hostage's turn. Edit: Had yet another rescue syndicate in Uranus today.  The hostage pathing AI will actually consider taking a shortcut by going in the water to reach me despite me not going in it at all.  Just being near a submersible hole is dangerous.