In LHTR, the attacker’s choice to retreat is made before the defender’s choice is made to submerge. In the box rules, the choices are made simultaneously.
how does THAT work?
Not very well.
Do you each write down your attack/withdraw perogative on paper and show it at the same time… I mean, how ELSE could you do it simultaneously?
That’s about the only way I can think of. As TimTheEnchanter pointed out, this is an ambiguity in the rules. There is no FAQ entry, and as far as I know no “official” ruling on this, and that’s why I answered the way I did. My personal inclination would be to follow the “natural” order of things and let the attacker decide first, as in LHTR. If anyone cares enough, I can ask Larry Harris, though I suspect his answer would agree with LHTR (for obvious reasons). Just one more reason to use LHTR rather than the OOB rules.