There's this organisation with an underwater naval base on Île Pott masquerading as a third rate hotel resort. Said base features several underwater hangars housing a considerable fleet comprised of several cruisers, destroyers, frigates, amphibious assault ships, submarines, patrol ships, corvettes, hospital ships, littoral combat ships, research vessels and aircraft carriers. All of these ships are designed specifically to completely submerge themselves underwater for long periods of time when travelling, in order to greatly reduce the risk of being detected and resurface upon reaching their destination.
One such vessel is the Kharybdis-class aircraft carrier, which can house about 50 manned aircraft, including fixed wing planes and helicopters on it's flight deck. Due to being incompetent with mathematics, I haven't figured out how big or long this thing is going to be. But, what I do know is that this ship will need a great deal of internal space to store these vehicles inside, some sort of mechanism to help it sink or float, and a hydrodynamic design that allows it to move efficiently on top and under the water.
How practical would such a vehicle be?