Think of a blow-up swim ring...and you wearing 40 pounds of lead....The only thing keeping you up is that ring.
Think of a mobile home-sized tube, inside that ring...and what would happen if something popped it, like say, a 5 inch diameter shell. Or worse.
Pressure hull? Pressure luck, because thats just to hold out the water if you have a good bouyancy hull, and ballast tanks.
Some folks just need time in a real-life swimming pool, and a blow-up ting to teach em about the vagaries of buoyancy.
Oh- don't forget: A sub, of itself, is an anchor....it's the tanks that keep it sometimes near the surface.
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