From the Lamannia Known Issues Marked as WAI:
"Hirelings
Hirelings will path to monsters across water if they think they can jump across. They aren't very good swimmers though, and if they don't make the jump, you will have to recall them to you."
Can I just ask one question?
Why do hirelings believe Lava/Acid to be Water?
I don't mind it being WAI that Hires can't Swim - I do Mind that they show the survival instincts of a recipient of the Darwin Awards whenever they come within 50 feet of Lava or Acid!
Can you please teach Hirelings that Lava/Acid are NOT Water and indeed should NOT IN ANY CIRCUMSTANCES be stepped in!