It's a pity though that this rule only applies to sea units and not land and air units.
IMO they certainly deserve a surprise factor too (after all: there were a lot of camouflaged fighters and tanks during WWII that seemed to appear from nowhere).
Correct me if I'm wrong, but in your rule you have to state that your hidden sea units are in a certain sea zone when the enemy enters that sea zone, right?
So you have to keep track of all your hidden sea unit movements (taking care that your opponents don't see you staring at a certain sea zone while writing) and also keep an eye on the 'empty' sea zones the enemy is entering.
Your movement notations can then be verified by the enemy, etc., etc.
And of course there's always a chance that two (or more) hidden fleets appear in the same sea zone at the same time, which is Oopsy Daisy to say the least.
The rule I use contains the same element of surprise, but without the writing and applies to every unit in the game (except industrial complexes).
You can surprise your enemy by stating there could be a hidden unit on a land/sea area he wants to attack (there's no fixed or written down area for a hidden unit to appear).
A reconnaissance flight can force the popping up of the camouflaged (hidden) unit.
Of course there are some restrictions to where and when the camouflaged units can pop up, but it's a rule that works perfectly well.
The hidden units are, visible for your enemy, placed next to the game board, but he doesn't know where or when they will appear until they actually pop up.
The whole writing down on pieces of papers is skipped this way and there are no discussions about what's written down (or not).