If it's big enough to hit, how does it avoid any of the onboard high-tech high-budget detection systems? I understand the oceans are massive. But if there are

If it's big enough to hit, how does it avoid any of the onboard high-tech high-budget detection systems? I understand the oceans are massive. But if there are 100,000 if these "structures" wouldn't there be better prevention? This are world class and world beating war ships, eh.