I don't understand from this why it is more dangerous to take off your fins before you get out of the water than on a swim platform in rough seas.
I think "swim platform" and "on the boat" are the same thing. The point I think is that until you are out of the water - and to me if you are on the ladder you are STILL in the water - you might not be done with your fins just yet!
If you slipped off the ladder you might end up a few feet away from it. If the seas are hopping you will definitely want to be! In a dry suit and double steel 119s it will be d@mn hard to get yourself back to the ladder, even in relatively calm seas, without fins.
This is of course if the ladder is a "fins on" type. If you must remove your fins to use the ladder, then that's another story.
Me? I would keep my fins on and my reg in my mouth until we're back at the dock and I'm getting into my car if I could!