They are in actively using, where convenient to block large water area. As even in war ports, usually active traffic. Second reason why now limited usage, WWII

They are in actively using, where convenient to block large water area. As even in war ports, usually active traffic. Second reason why now limited usage, WWII submarines was very limited power, many experts consider them as just high-speed boats with extremely limited underwater capabilities, but modern submarines are really powerful, especially nuclear.