Assignees are currently ignoring this rule
If that's the case, why don't they begin Swimming once the assignment has been applied?
My guess is that in general, if an Assigner gives them a Swimmer skill, no check is done to handle "if already drowning, transition to swimming" (and thus if you could set this up without the ohnoer being involved, you'd still get the same result).
As far as I can tell, in order to trigger this (once their interaction with ohnoers is fixed) you'd need an extremely specific setup: A teleporter overlapping water, with a receiver that overlaps an assigner. Send one lemming through the teleporter while a second slips past at the last moment, just enough so to start drowning before the teleporter catches him. Now with that being said, I would argue that the logical behavior here is not "prevent the assignment", it's "make the assignment rescue him from drowning, as it would with a manual assignment".
