@beelee

In the 'North Africa' game, the map has silhouettes where navel units can be placed. The Americans can only deploy the Destroyer in Sea Zone 2. A hidden factory can be placed in their Casablanca Convoy. A hidden factory in Gibraltar for Sea Zone 3 and the Atlantic Ocean Convoy for the British. A hidden factory in Italy for the Italians for Sea Zone 10. For the German Submarine a hidden territory with a factory in Sea Zone 1.
By resetting 'unitPlacementOnlyAllowedIn' prior to each player's placement phase, navel units can be restricted to those sea zones.

Cheers...

P.S. Using hidden factories on hidden land/island territories can also help in the placement of Sea Mines. Prior to placement those territories would change ownership.

Again.

Cheers...

yea Idk how you can change ownership though. If the Brits have the TTy then the yanks can't use it. Maybe a seperate box for each player and you can place the dudes there. The Box is connected to the SZ.

It'd still burn a movement point. You could at least edit then. You'd have to edit anyway but it might be easier being able to at least place the units then edit move as needed.

Or maybe the Box could have a invisible permanent +1 move unit to all units.

Edit
The Box is connected to all SZs, so it'd only be one move to go to any of them

Edit 2
Hmm if the SZ can be bordered by multiple TTys then you could have multiple Fctrys of different Players using it. Surrounded by water though. Idk. you'd have to make mini TTys that border it.

Be pretty sloppy. Better to just use edit in that case. Might be able to get some of the TTys though

@victoryfirst

For 'placementAnyTerritory' and 'placementAnySeaZone' the territory/sea zone needs to be owed by the player placing those units. Territories are owned by the player that last pass through. Sea Zones are considered Neutral owned, unless units are present. This is why navel units can only be placed in a zone currently occupied by another unit.
Basically @TheDog has your answer. Use invisible factories and limit placement with 'unitPlacementOnlyAllowedIn' to limit the sea zones. If needed, 'unitPlacementOnlyAllowedIn' values can be changed with triggers.

Cheers...

@VictoryFirst
You probably need some invisible factories on the coast of where you need to place shipping.
With code that only allows ships to placed.