(Yes, this is the poster formerly known as TheWatcher but for some reason the board refuses to let me log in as that any more)
The problem with naval levels is that to work they have to be pretty big. Very big if you're playing around with destroyers, stealth jobs or not. Most FPS engines are absolute rubbish at displaying very wide open areas, although I'm not sure whether the newer revisions of the unreal engine are as troubled by this..
I'd be surprised if the DX engine could handle anything like that at a decent speed