An alternate method involves using bidPlace delegates. If you want to place one sea unit per player turn in a turn order. You will need to place the multiple delegates before the regular sequence begins. The trick here would be to trigger one sea unit to place (during the bidPlace) and additionally trigger an invisible sea unit (owned by nation whos turn it is) into every available sea zone during the bidPlace and remove all of them after the bidPlace. This will allow you to place sea units in sea zones that look empty or unowned.
But if you actually require ownership of the seazones to be changed, you need to trigger them as well. Or try assigning each seazone and attachment before hand that has ownership assigned, and see what happens. But I think triggers would still be required here. Maybe try adding production=0 to seazones too, then it should switch ownerships.