Would love to see subs int he game, but with the water being 0 altitude, I'd imaging that you'd have to get a new base line or open up negitive numbers. Which usually dont work very well in the computer world. I could be wrong though. If they were inplemented though, Id love to use it for base defense. 
Actually, negative numbers are used quite often when programing games, especially in FLASH. It's usually used for movemement, for example KEY.isDOWN(left)
{
move = -move.x;
}
That should be self explanitory, but just in case... If the left arrow key is down, then you move along the X axis negative (left) amounts. You could also use rotation but then that opens up for some more complex stuff with radians and all that fun jazz... stupid car racing games, take forever to do just cause of how many lines of code there are, for not that much reward. But then again, that's programing.
The problem would be implementing it...