That's why I mentioned sneaking. How many times in ARMA2 have you seen AI sneaking?
EDIT:
What I meant to say that it would be very hard to pre-program AI for actions under water. When would it be practical for AI to jump into water? When to dive? Maybe hide behind a rock and return fire be better? Now it's a lot easier to pre-script an event where the AI comes into action from under water (AI actors spawn under water, come close to the shore and get out). If they do actually manage to make AI to flank the player(s) and use water for flanking (which is a not that useful in a military situation as it brings up sound and reduces visibility, especially if you are under water and have no intel). This makes other options more preferable, which will force the Automated Intelligence to choose something more efficient rather than try to improvise.
It is similar with sneaking. Humans are good at spotting, especially in computer games, so sneaking for the AI becomes not necessary plus very hard to configure. It is very hard for AI to bring the element of surprise as there is no real way of implementing improvisation or situational creativity.
But when human players clash and you have spotters and a flanking group under water, that's where things will get into play.