Dont matter if the Ai subs 'cheat' to determine depth,
The Escort/merchent AI cheats all the time anyway,
One example is that they can see you though land mass, and look at the speed at which they can 'stop' or accelarate when they need to avoid collision
Anyone remember the AIs 'Vampire vision' at night?
It doesn't need to be perfect, just so long as its THERE and works to a reasonable level.
- if need be modders can tweak it.
Id rather have AI subs that crash in to the ocean floor at shallow depths - than none at all.
At least you will get to see them working properly far out to sea (where the player will mostly be operating anyway.)