I've spent a disturbing amount of time on this so far, as you know since we've been talking about this in PMs. And no, there's no simple XML fix for this, although from what I can tell it'd be trivial for the devs to add one if they wanted to.
Also, this is in the wrong forum. It should be in the base C&C forum, these subforums are only for mods with actual files involved.
1> There are three Lua events that matter here:
SerialEventUnitCreated triggers when a unit is created, embarks, or disembarks. There's no easy way to tell which of these three called the function. I've gotten around that by adding a "Rookie" promotion in my mod, which all units get at creation and is removed as soon as the unit is in a fight. So through a combination of checking the turn number the unit was created on and checking the presence of this promotion, you can tell whether the unit was just created. (If you simply never remove the promotion, then there's no need for the turn check.)
One upshot of this event is that you can remove the free Embarkation promotions easily when a unit is created, to ensure that it hovers over shallow water.
UnitEmbark triggers when a unit embarks or disembarks. Three arguments: player ID, unit ID, and true/false (true when it embarks, false when it disembarks). Unfortunately, if you try using the Units:Embark() command inside here while the unit still has its embarkation promotion, you'll get stuck in an infinite loop.
There are several Lua events that trigger when a unit moves; most have the player ID and unit ID and nothing else, but a couple also pass the X and Y values (although you could get those from the unit's database just as easily). The best of these is the UnitSetXY GameEvent.
However, all but one of these events will only trigger if the destination hex is valid, which doesn't help here as what you really need to know is when the unit WOULD move into the impassable deep ocean while hovering. The only one that does trigger for an invalid move (StartUnitMoveToHexes, I think) has no arguments at all, not even unit ID.
Now, you could still get around this through the SaveData structures, and I'm working on just that. Basically storing whether that last event triggers without the other ones, to know whether an invalid move was ordered. Unfortunately, there's no way to know what hex the unit was moved to order to, without also trapping the mouse position or something, and that's not going to work for anyone other than the human player.
What we need is a modified UnitSetXY that reports when an invalid move is ordered as well, with full positional information. But this wouldn't help in the long term, since the AI would simply never give those invalid orders.
2> For XML, you'd have to go about things a bit differently. It IS possible to make a true all-terrain promotion, by doing the following:
> Go into the database and change Ocean and Coast tiles to be land instead of sea
> Make all naval units be land units with the "Naval Unit" promotion, making everything other than coast and ocean impassable.
> Make all land units start with the "Land Unit" promotion, making coast and oceans impassable.
> For the all-terrain units, just don't give them either of those promotions. (Although the CanMoveImpassable might override them anyway.)
As you can tell, this'd be tremendously messy, as it would basically render the Domain system useless. You'd have to rewrite all of those "Naval Penalty" type promotions to key off of combat classes instead of domains, and the Barracks and such would now add XP to naval units. On the bright side, the AI would know how to handle this.
3> There's a possibility I've also been looking into: cheating. Basically, let's say I made a "Gravtank" unit that is intended to work this way. So I create a "Gravtank2" unit; exact same stats in every way (including the Armor combat class), except that it's DOMAIN_SEA instead of DOMAIN_LAND. Neither has Embarkation.
So when the Gravtank moves over water not adjacent to shore, the game spawns a Gravtank2 on the same hex, gives it every promotion that the original had, sets its movement to whatever the original had remaining, and kills the original.
In reverse, as soon as the Gravtank2 moves to a hex adjacent to land, it switches back to a hovering land unit through a similar process.
(This could be done even easier if the game provided a Units:ChangeDomain() function, but it doesn't.)
The problems:
A> The naval combat penalty. Naval units have much weaker stats than land units, which is why so many things have the naval penalty. Gravtank2 could have a lower combat rating to compensate, like half as much, but it'd now be far more vulnerable to artillery.
B> 1-hex-wide continental shelves.
The above method only works if the shelf is 2 hexes wide, or else you can get situations where the unit just doesn't trigger the transformation.
and the big one:
C> The AI would have no clue how to do this.
It would think the ocean is impassable, and so not try to move to that continent on the far side. It's possible that the unit might hover too far from land, transform, move across the ocean, accidentally come up next to the shore and transform back. But it's horribly unlikely, which means this'd be a very unbalanced system.
This last one is the major drawback of nearly any system you can come up with; doing something the game already knows to be invalid just prevents the AI from using it, so they're not good long-term solutions.
So until the devs release the DLL, I'm not going to go much further. Right now, I'm focusing on making a poor man's version of this by giving all units with this promotion two things: the defensive embarkation of the Songhai, and a triggered Lua event where whenever a unit embarks or disembarks it refunds the movement points that were lost in the process, allowing the unit to keep moving through the water. It's not as good as what I'd wanted, but it at least is something the AI should have no issues using.
Also, I added airlifting in my mod (through a custom paradrop promotion that only appears when you start your turn in the city with that building), so having these units be unable to embark isn't crippling.
Movement Points) causes the unit to be able to move an indefinite number of tiles and makes it to jump back to the closest city at the beginning of the player's next turn, which is not desirable.
That'd just be fantastic!
