The advantage to hiding a giant ship underwater is that it can sneak up on targets undetected. The disadvantage is that’s it’s hard to keep tabs on what’s going on above the water line: a problem a company called SpearUAV has potentially solved with a quadcopter that can be launched from a sub while it’s still submerged.
The earliest submarines mostly stayed near the surface of the water and would dive only to escape immediate threats, but the advent of nuclear-powered subs meant the ships could stay submerged for prolonged periods. It allows them to operate in stealth, but also limits their ability to communicate with the rest of a country’s navy and access modern military tools like GPS and satellite imagery. Wireless communications don’t work through salt water, or have a very limited range. So if a sub captain wants to do reconnaissance above the water line, they need to surface the ship to deploy tools like periscopes or wireless antenna: potentially revealing their position and putting the ship at risk.
SpearUAV’s Ninox 103 UW sub-to-air drone is a safer way for a submarine to peek out of the water, and takes advantage of the autonomous flying, obstacle avoidance, and navigation capabilities of modern quadcopters, which can often take to the skies for almost an hour, even while carrying camera equipment and other sensors.